If Russia and America full sent against each other, America would absolutely full send against China irrespective of what China did or didn’t do. Hell, they will even full send against the Indians, because if they are getting wiped off the face of history, they will be damned to leave anyone else to inherit the earth. That’s just how they are, and it’s proven by their own declassified Cold War files.
This is also why nuclear escalation is such a red line for China in Ukraine, and why China will do almost anything to prevent it, since they are getting dragged into it if it went nuclear no matter what. Better to keep things from spiralling totally out of control with nuclear exchanges that will almost inevitably lead to total global nuclear MAD.