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

[Del Tachi]

I question your premise.  When has any nuclear power demonstrated, even rhetorically, a willingness to use nukes against civilians?

The goal of any first strike would be to take out your rival's (1) command and control centers (i.e., so Washington gets hit but not Orlando) and (2) reduce the capability of your rival to retaliate, which means hitting military targets like airfields, missile silos, and navy ports.  Below this first tier are secondary targets like power plants, communications hubs, and other major civilian infrastructure.  Taking out these facilities degrades your rival's military indirectly by disrupting their supply chains, communications, mobilization, etc.

A lot of civilians would certainly be killed in these attacks but that wouldn't be their purpose.  There is no strategic advantage gained from leveling Midtown Manhattan, and brazenly attacking civilians needlessly risks escalating the conflict beyond control.  Attacking cities only makes sense as an end-game maneuver to diminish your rival's long-term ability to rebuild and recover after their military has been destroyed, but at that point your armies have already moved in and taken control for themselves.