most likely starting point for WW3 is a border conflict between china and india imo
second most likely would be china trying to invade taiwan, and japan + USA decide to step in to prevent it
third most likely, probably something related to russia overreaching in europe (baltics) and triggering NATO retaliation
middle east could go up in flames in a major regional war (if iran and saudis get involved) but i don't think the big global powers will get dragged into it, at least not beyond half-assed proxy war shit like we already saw in syria.