Allow me to state my point:
-During the 1939 battle of Kalkhin Gol, the second rate Soviet Army of 39 decisivly defeated the Japanese imperial guard.
For an introduction one can consult the work of David Glantz or Edward Drea on the subject, f.e.
In 1945 the Red army, having finished off the Nazis in Germany arrived in Manchuria and handled the Japanese Kwantung army like a red headed stepchild.
However, I am not particularly happy with the whole arbitrary "Big three" thing in general.
Imho, Russia was by far the MVP on the allied side, and Germany was by far the heaviest hitter on the axis.
Beyond putting Russia on place one, it imho becomes very difficult to assess each allied members contributions. China had a very large contribution in manpower, contributed this manpower for a long time and did keep a lot of Japanese formations occupied.
In this they likely influenced the decision of Japan to not invade Russia, but I would argue that the battle of Kalkhin Gol did more to that. The internal dynamics of the Inner Japanese power struggle between Admirals and Generals are of course another issue.
Of course, one cannot completly see Kalkhin Gol in isolation from the Sino Japanese theatre, and it may have been that the Japanese could have brought more forces to bear had Chinese resistance been weaker, a different outcome of Kalkin Gol may have changed history more than most people would expect.
If you wish to "rate" military contribution, it could perhaps be interesting to imagine scenarios which set that military contribution to near 0.
If we imagine that Chiang either folded quickly, or folded before Russia gained control of the east front, and a Japanese offensive into Siberia coincided with Barbarossa, things would look very bleak for Russia (who is the allied MVP in terms of military casulties caused) and as a result the allies.
In my opinion, things would be roughly as bleak as if the UK had folded early, both scenarios could feature enough axis reinforcements (either more Germans in the East front, or Japanese in Siberia) to tip the scales in the Axis favor.
If you set the USSR to 0, (a scenario in which the USSR collapses after the Germans "kick the door in"), earth is pretty much fucked. I seriously question the ability of the Americans to invade either Germany or Japan with the USSR in German hands, and it would also be incredibly bad for any Chinese resistance.
If you set the USA to 0 (imagine a scenario where they dont oil embargo the Japanese and thus no pearl harbor happens, neither Germany nor Japan had the ability to actually force them out of the war, so USA staying totally neutral), then the question becomes: Can the USSR win the eastern front without US supplies (imho yes but would take longer) and can the UK and China prevent Japan from opening a second front in Siberia (imho potential to do so exists) by fiercily resisting Japanese aggression.
If you look at it from a more conventional point of view, military contribution would rather strongly be corellated with succesfully defending ones territors, making counter offensives against the enemy and eventually planting your banner on the other sides seat of gouverment.
China did, with more success than most people in the west think, tenaciously defend herself against Japan, but it could not go on the offensive. In addition, it tied up 3 million Japanese rather than killing/capturing them.
The USSR and the USA did take the offensive and eventually won.
Imho, Chinas contribution is roughly on a level with the UK, above France but below the USSR and the USA.
I would, for "emotional" reasons argue against putting France in the same level as China. When China was attacked, it was facing a highly complicated internal situation, suprised, and quite strongly "outteched". However, it did not fall. France, in theory having the best army of the world did fall.
China performance was imho what one would/could expect of it, France was below expectations.
