I think improvements in laser and shield (yes, I said shield) technology will make nukes a think of the past. However, if WWIII does come to pass agree that it will pit the US and Europe against a pan-Islamic alliance united under a Caliph or against the PRC. (In the case of the latter I see Taiwan or North Korea as possible boiling points.) Russia and India could go either way, I don't think India could pass up the opportunity to strike at a weakened Pakistan and Russia would probably hit China if they weakened their garrisons to fight against America, India, Europe, whomever. Either way the United Nations will have almost no affect on the war, if it is even still around at the end of said war.
Africa and South America, well for some reason I doubt they will have much of a impact on any future war, of course tht is assuming that nothing changes between now and then. Which in all honesty I don't think is assuming too much.