Another flashpoint between Russia and China could well be the Russian Far East; Russia has long been pretty paranoid that China is going to try to claim the land as its own.
Didn't Russia and China settle their border differences in early 2000s?[2] [1] https://en.wikipedia.org/wiki/List_of_projects_of_the_Belt_a...
[2] https://en.wikipedia.org/wiki/China%E2%80%93Russia_border#Po...
I can see that changing quite quickly if there's evidence of a political shift in Central Asia that disfavors Russia. Say a new Turkmeni strongman decides to start his foreign trips by visiting Beijing and doesn't call on Russia for a few years. Or a Putin-successor with flagging popularity sees being more combative here as an easy way to boost popularity (much as the Crimean annexation was driven in part by Putin attempting to improve his support)--although targeting Western country is more likely here.
> Didn't Russia and China settle their border differences in early 2000s?
The lack of existing border disputes doesn't mean that the population accepts the border. Take Alsace-Lorraine--the border was never a live dispute, but France and Germany sure saw fit to switch it to their side every opportunity in the early 20th century. The Chinese have their own self-image as being a nation unjustly carved up and tormented by foreign powers, and the current border is seen as being one of those examples [1].
To be clear: I don't see any of these flashpoints as likely; I'm just pointing out that there is a split between Russia and China that could lead to a war, and that war between any two of Russia/China/USA is plausible.
[1] https://www.scmp.com/week-asia/geopolitics/article/2100228/c...
I think actual war is unlikely between any two of these three but I think skirmishes are very likely. Russia and China/US I doubt as Russia doesn't seem to have a concern which overlaps with the interest from other two on ground except Syria I guess, but between US and China, South China Sea might become a flashpoint if they both keep pushing each other there out of ego.