Cue essay:
First off, the OP is more than a little ridiculous. Throwing fantasy tech to shore up one side in what you think is a lopsided match defeats any valid purpose for such an exercise, all the more so because it's not as lopsided as it appears at first glance. A more appropriate thing to consider would be something that could actually exist/occur that would affect the dynamic of the fight. And make no mistake, there are scenarios where Japan wins against both. But that's the clincher: who wins depends on how and where such a war is being fought.
First, let us understand the players and their capabilities:
Japan
Japan's military was designed with two roles in mind, and two roles only. It was intended to defend the home islands and Okinawa against a potential Soviet invasion (as part of a larger war), and it was intended to protect air and sea lanes in such a conflict. Due to Japan's geography, there are only a handful of viable landing sites, so the disposition of ground forces was basically a foregone conclusion. However, due to the pacifist requirements of both its constitution and its people, Japan built and maintained very little logistical infrastructure, and is thus largely unable to deploy large forces to other areas in a reasonable time frame. Amphibious warfare capability is also very limited, as that is considered offensive in nature (even though it's necessary considering Japan is a nation of islands).
Japan has a very high level of military industry, and develops and produces most of its equipment itself. In fact, it overpays, using military contracts as a form of corporate welfare to preserve its manufacturers. This does result in some claims that the equipment may be "below spec" in many cases. While it needs to import resources, no major war is likely to last long enough for anything other than oil to be exhausted.
The military has, since it was re-created, almost never met its authorized personnel thresholds. As such, units are technically understrength, and many soldiers have chosen the military as a temporary job in order to take advantage of the benefits. This, combined with antiwar politics has resulted in some embarrassing situations in international peacekeeping missions that has left the Japanese military the brunt of many jokes. While professionally trained, the willingness of individuals to fight may be in question in many circumstances.
Japan's combat air arm is very modern, but almost purely dedicated to air superiority, (defensive) naval strike, and direct support of ground forces. It lacks major transport assets, and has limited inventories of standoff ground attack munitions and refueling assets, so it will mainly be limited to protecting Japanese airspace and territorial waters. To that end, it has excellent surveillance and control assets, with 12 E-2s capable of tracking not just planes, but surface ships as well, and 4 E-767s, which are more modern than current US AEW aircraft. This allows them to generate up to 5 simultaneous surveillance zones, or areas where they can provide airborne radar coverage 24/7, and temporarily surge that to 6 or 7.
The JMSDF (navy) is actually arguably the second most powerful in the world. While lacking true carriers, it has more modern destroyers and frigates than any nation in the world save the United States. And while lacking nuclear-powered submarines, it has a very large conventional submarine force with top-of-the-line units. Its amphibious and supply assets, however, are laughable - almost nonexistent.
As a result, the military is very modern, well trained, and very well equipped. However, it is sorely lacking in logistical infrastructure, particularly the capability to reposition major units, and especially those with heavy equipment. And, well, let's just say they're not likely to go at it tooth and nail like Israelis, except maybe in defending the home islands.
China
China has a very large military and territorial ambitions - not unlike other nations in the past, they believe history and their perceived superiority dictates that all territory with desirable resources are theirs, and they merely need to wait until they're strong enough to take and hold them. As a result, Japan has current territorial disputes with all but two countries that share a land or maritime border with it. In short, they actually need to maintain a military presence along most of their borders. However, they lack the capability to concentrate their forces and are far from overcoming the huge logistical barriers their nation's size, geography, and population presents. Their weaknesses became painfully apparent in the invasion of Vietnam in 1979, which prompted measures to be taken to fix this. 35 years later, they still haven't succeeded - only a handful of units can deploy to new locations on short notice. China has continuing disputes with almost every neighboring nation (Pakistan & Russia being the exceptions), and therefore must maintain strong forces throughout the country, including the need for modern units in the south and west due to India's strength.
The ground force is large but only moderately well equipped. As with all branches, most of the units are a generation or two behind western counterparts in capability, though the gap is closing. Of greater significance is that, unless Japan invaded China, only a small portion of the PLA ground forces would be able to participate in a war. China has only a few divisions that can be redeployed with any speed, and just a few others intended for air assault or amphibious operation. These are aimed at Taiwan and the South China Sea, and reorienting them against Japan would be difficult, time-consuming, and easy to notice.
The PLAAF is also heavy on numbers, but less so on equipment. Over half its combat aircraft are obsolescent, and due to other commitments, Japan would actually maintain at least parity, and quite possibly a genuine advantage, in numbers of modern combat aircraft. This will hold for at least the next decade, though China is producing increasing quantities of increasingly capable fighters. However, its AEW assets are quite light for its size. I estimate a capability to maintain only 4 simultaneous surveillance zones, with a surge capability of maybe 6. However, it would need to at least double its assets before it could attain parity with Japan on this front, as it still has 4 other places it should have them (Taiwan, South China Sea, Kashmir Region, and eastern border with India). Their transport fleet is sufficient to support brigade-sized airborne operations and could, though at great cost, ferry 1 tank battalion or 4 mechanized infantry battalions at a time, but would be hard pressed to support such forces via air.
The PLAN has actually almost caught up to Japan in sheer number of major surface combatants, but again must maintain significant forces arrayed against Taiwan and in the South China Sea, where China's territorial ambitions are most pronounced and contested. They do still have a large coastal force that could range far enough out to engage Japanese warships in certain situations, but those are again largely scattered. China's current, and projected future fleets are only about adequate technology-wise. While they have carriers in the pipeline, these are still a long way from being fully functional, and are not quite the game changer locally as carriers are mainly for projecting power to distant locations, not dominating nearby ones. They've built a few area air defense ships, but don't like the prohibitive cost, and only use somewhat dated Russian and French missile designs, often with less-than-spectacular command and control. Their antiship missiles are better, with the most common ones on par with the US and Japan with a handful of more capable units. Their submarine force is good but not great. Only the newest home-built vessels and their Russian imports are really that useful, as their nuclear submarines and most of their locally produced conventional ones are a generation or two behind Russian tech, which is itself a generation or two behind western. They can put up a good fight based on numbers alone, but would be hard pressed to win an offensive naval war against any major western nation without preparation and good land-based air cover. China does have a significant and rapidly improving amphibious force, sufficient to deploy two reinforced brigades. However, most of its large landing ships are the LST-type that has to drive all the way to shore (easy targets) and lack provisions for supporting those troops long-term. This is getting rectified over the next decade, as the number of LPDs will at least double, but China is still over a decade from being able to fully deploy and support a division-sized force, and would thus have to focus on securing a port quickly.
I don't think I need to go into heavy detail about the capabilities of the US. Suffice to say they're the only nation on earth that can have an entire air force and several divisions anywhere they're needed in a matter of days, and have well trained and well equipped troops.
Likely scenario 1: Chinese attempt to seize Senkaku Islands
Since the Senkaku Islands are very small (just over 6 square kilometers between all of them, and only 1 much more than 1 km2), ground forces are of no consequence. Even Japan's miniscule amphibious assets can land far more troops than the islands can support. Instead, it would be a naval and air battle. Each side has advantages here. China has a major defense concentration here as the Senkakus are near Taiwan, and the distance is close (330 km), so aircraft need not fly far to reach engagement zones. Japan, on the other hand, is hurt by the fact that its nearest major military facilities (Okinawa) are about 25-33% further than China's, and it's assets are concentrated mainly far to the north among the home Islands. That being said, they will be able to deploy at least some assets to Yonaguni and Ishigaki Airports, which are only about 170 km from the Senkaku Islands. This one becomes a toss-up with numerous variables that could tilt it either way. China could win an aerial war of attrition or suffer unacceptable losses from Japanese air defense destroyers. The PLAN could overwhelm the JMSDF with large numbers of small fast attack craft, or itself be decimated by the JMSDF's superior C3I capabilities or even just the submarine force. For the forseeable future, this could just go either way.
Less Likely Scenario 2: China goes after Okinawa
This, of course, assumes the US pulls out, and is not as far-fetched as it sounds. There has already been some quite "historical claim" talk, and that's how this kind of dispute tends to start when China eyes a delicious morsel. Now, at present, there is little or no armor stationed on Okinawa. If the US moves out and the Chinese start looking that way, expect it to change, but China can probably mobilize and invade faster than Japan can relocate an armored battalion - unless they did so before the US finished leaving. Assuming they manage at least temporary local superiority. Option 1 has the Chinese navy getting badly bloodied and beaten to a pulp by maritime strike, coastal defense, naval, and submarine assets - all benefiting from Japan being the defender. However, if China manages local superiority, they will be able to deploy maybe 3 brigades between airborne and amphibious forces, against what would be either an understrength brigade or a reinforced brigade combat team. Okinawa is close enough for both sides to fly air missions from their mainland bases, and Japan has a slight advantage in long-range combat aircraft. Again, this could go either way. It depends on the losses China takes as it tries to land and support landed forces, whether China can take over local airfields and ports, and how prepared Japan was for the invasion. This one does, however, edge heavily toward Japan. China is decades from being able to seriously threaten Okinawa. The other southern islands are a different story, but Okinawa's a tough target.
Somewhat Likely Scenario 3: China expands a war and goes after home islands:
This is possible but unlikely. China would not want this and would not do so unless it was 1) a temporary diversion to pull forces away from the main target or 2) they got desperate. The closer to the home islands a battle gets, the further from the major Chinese military concentrations and the closer to the Japanese ones. Designed to fight off a joint Sino-Soviet invasion, a Chinese invasion of mainland Japan would be almost as disastrous as a Japanese invasion of mainland China. China may inflict damage, but Japans air and naval defense assets would decimate Chinese forces, and any ground troops that landed would likely be slaughtered, especially if civilians got harmed. This scenario could only be a Chinese victory if the operation was a diversion and it succeeded in that respect, allowing more distant targets to be taken.
There is no seriously viable scenario for a US intervention. I mean, we could imagine an autocrat taking control and committing atrocities, but more likely the entire military would rebel and unseat such a person. Japan currently lacks the capability to take any territory due to lack of amphibious assets, and the only two pieces it really disputes that it doesn't control are Dok Do (controlled by South Korea, who can defend it themselves) and the Kuril Islands (Russia). If a battle was waged outside of Japan, the US would win. The US could beat any 3 nations if the site was well beyond any of their borders. However, an invasion of Japan could fail. Not necessarily would, but could. But again, it's a bit out there to contemplate.
