A country would have to do something very stupid for there to be a full on 'World War 3'... No one really wants to go to war, and it is unlikely we will see a conflict as large in scale as world war 2 any time soon. There are regional conflicts that could potentially exacerbate tensions between superpowers, but the concept of mutually assured destruction is a pretty damn good deterrent against a full on war between nations.

In my uneducated view, a 'world war 3' would necessitate a large amount of the countries on earth to 'take sides' on a given issue that they deemed worth fighting for. The worst thing I could see happening would be escalating regional conflicts and an ensuing 'proxy' war. Russia and the US have a keen interest on the outcome of Syria, and are both providing support to various factions. Let's say a US led airstrike in Syria leads to the deaths of a bunch of Russian troops in the country. The worst response I could imagine would be Russian military strikes on US defense assets in the area, which would lead to a conflict confined to that region. That would be a giant clusterfuck, but I don't think that would lead to a direct conflict between those two countries. The second Russia attacked the US mainland, and vice versa, nukes would fly and the very course of humanity would take a dramatic downward turn.

TL;DR: In a full scale war, it is now too risky to attack another country without enduring terrible consequences. When you have land, air, and sea based capabilities, country A could launch their entire nuclear arsenal at country B, and country B would assuredly have enough of an arsenal remaining to decimate country A with the remainder of their nuclear arsenal.