If WW3 happens then it won't be a war as either WW1 or WW2 were - the world has changed so much since then. The initial fighting would be confined to the relevant geographical location for as long as outside forces can manage, but the nature of war - particularly in this modern age with conflict in so much of the world - means the fighting would spread out from there sooner or later.
Best case scenario for either a Russian or Chinese conflict with the US (no, very recent history does not rule these out as being serious possibilities) :
- http://time.com/3934583/world-war-3/
But really, it's not (just) the actual fighting to worry about :
https://www.businessinsider.com.au/president-nuke-option-requires-no-permission-2017-4?r=US&IR=T
- - - Post Merge - - -
Unfortunately that is not the case. Even if the main conflict occurs elsewhere, there would be continued attempts to bring the conflict to the west, particularly to the US. It would not be an actual war otherwise.
Furthermore, the rate and scale of terror attacks (from all sources) would almost certainly increase. Terror thrives in chaos, and nothing is so chaotic as the threat of war.
I want to end on a more positive note but.. I'm at a bit of a loss for an upside at the moment.
With the fact that the world has evolved so much since the first world wars, wouldn't you think that there would be no point to a nuclear war? Wouldn't it just basically destroy everything and nobody would win? Why can't these leaders see that?