If Pakistan and/or India start getting nuclear on each other's asses, I don't think that would trigger World War III. It would be ugly, yes, but I could see there being enough restraint among the remaining powers to, pardon the pun, "go ballistic". It's not like either country has that big an arsenal to begin with, anyway. As for North Korea, that's also questionable. Given the limited size of their arsenal, if they were to use it in anger, they'd probably use it The Sum of All Fears-style in a sneak attack for maximum impact.