Nobody is using any nukes unless they're crazy. Terrorists taking control of pakistan and using them is the ONLY way I think nukes would be launched.
Nuclear threat had its hay-day during the cold-war. During which, neither countries seriously considered a preemptive strike, or a first strike.
In fact, in a book I am reading about the cold war(George Kennan, Paul Nitze and the history of the cold war), the russian secretary of state was visibly shaken, pale white, and jittery during a real ICBM(dud nukes) missile response drill. Repeatedly querying about "real world consequences should he press the button."
The ONLY person to suggest a preemptive strike was Castro, and was immediately denied.
You have to be CRAZY to launch a nuclear missile at any country. If you do it, you're as good as dead, especially if it is a U.S./Russian ally.
Anyway, I doubt world war 3 will happen truth be told. No country is really in the position to any time soon.