If some system somehow makes nukes impractical or impossible to deliver against one's enemies, then we're looking at a real possibility of WW3. Until that time, the 7 nuclear nations dare not fight each other. Without those nations, its not much of a "World War".
One curiosity. Did Russia take control of all the nuclear warheads of the former USSR?? I'm surprised if they did. The Ukraine, Belorussia and others would have surely fought to retain possession of some of them, especially those nuclear weapons that were still within their borders when the USSR was dissolved. Same goes for Kazakstan, Uzbekistan, Turkestan, Armenia, Georgia .. etc. Its particularly important to remember that a huge number of nuclear silos and weapons were concentrated in and around Kazakstan. For the time being, most former Soviet states (not counting the Baltic States) cooperate with each other, but that doesn't mean they always will. How sure are we that Russia got every single nuclear weapon?