If it actually came down to it, would USA/Russian/China nuke major cities, or is that bluff?

[Agonized-Statism]

Not unless this is it and everyone feels the urge to launch everything they've got, in which case the priority would still be hitting the enemy's silos over their civilian populations (you've got better chances surviving in California than you do in North Dakota). But yeah, cities would get hit. That's the theory of total war, civilians are part of the war effort and therefore valid targets. It's pretty awful. That's why John McCain was the most dangerous Republican candidate this century.

I could certainly see Russia or the US using tactical nuclear weapons under pressure and escalating from there, hitting civilian centers in the country they're invading. In some other world, John Bolton got to play MacArthur and North Korea or Iran is radioactive.