I don’t think we will ever see a World War 3. At least not under current definitions.
The World Wars are called such because practically the whole world was involved in Total War. Every involved country geared its entire effort towards the war, and massed troops faced off all over the world. This is not possible in the modern world for a number of reasons.
1. Nuclear weapons. If any of the nine nuclear capable countries are too hard pressed in a conventional war, they will escalate to a nuclear war. While the threat of nuclear weapons is now a necessary part of the global power balance, no country would be stupid enough to back a nuclear power into that corner.
2. Economics. Modern weapons are expensive. If a country does not have a strong economic foundation, it cannot afford to wage war. If a country does not have good trading partners, it does not have a strong economic foundation. Very few countries can afford to wage Total War anymore, and even those that can wouldn’t last long before their people started to starve.
3. Civilians. In both the World Wars, civilians pitched in, and made ammunition, radios, and anything else required for the war effort. But modern weapons require greater expertise. You can’t take an average civilian and teach them how to assemble a laser guided bomb in a week. Therefore a country must keep the majority of its civilians in civilian roles, which reduces their capacity for war further.
4. The changing nature of conflict. It is possible, in our modern integrated world, to sufficiently weaken a country without warfare. One needs only look to Venezuela and North Korea to realise this. Both countries are politically and economically isolated enough that they couldn’t sustain a major war for more than a few months. Neither have been brought to this point by war. Stuxnet also showed the efficacy of cyber attacks in decreasing a country’s capacity for war. If a consensus forms that a country should be weakened, it can be done without war. Not to the point of capitulation, but at least to the point of deescalation.
At the same time, we’ll never have world peace™. We will continue to have proxy wars and regional conflicts. We will continue to have escalations and chest beating. But I don’t believe the homeland of a major power will ever be threatened again.