They aren’t getting ready for world war 3.
China isn’t going to go to war for Putin.
At the most this ends up with more Russian boys becoming cannon fodder to appease Putin’s ego.
In practice, many of the Russian reservists will not report or will be found to be unfit or exempt for some reason. Their training will be stale, and weapons and equipment will have to be scrounged or repaired and refurbished. Due to the Russian need for an immediate injection of manpower, most of these reservists will simply be fed into existing units that have been thinned out in combat, been defeated, or otherwise collapsed in battle. That may fill the ranks, but without time for training and without new weapons and equipment will do little to rebuild such units into militarily effective formations.
In sum, Putin's partial mobilization will not reverse the adverse course of events on the battlefield but seems likely to erode his political base as Russian families lose more sons in a dismal and failing effort. NATOs members will continue to support Ukraine, and likely with a sense that Putin and Russia will soon be played out in military terms.
Maybe not WW III if it has to include China. China is playing "Let's watch you and him fight." It would get to pick up the pieces of Siberia it desires, and have no significant rivals after a true West vs Russia war.