World War Three probably won't be fought with nuclear weapons. The widespread use and drawbacks don't make them viable options for a country or planet intending to survive WW3. That having been said it will be unlike any war you can imagine. Biological weapons on nuclear scale which is to say hundreds of square miles of bioweapon fallout and destruction are barely the beginning. Animal and plant life will be genetically modified into ultra deadly weapons that go unnoticed by security systems and cause chaos behind enemy lines. The war will be fought on multiple fronts, including a digital one in which access to and delivery of critical information particularly to the front lines will be determined on a regular basis. This can make it so operations are carried out on bad information or the lack of information cripples squads and leaves them without support. Even the support such as air, tanks, and artillery may attack the wrong targets or move to the wrong locations.
The consequences of the advanced weapons will cause the air itself to become hyper acidic destroying all it comes in contact with. Advanced robotics systems which are fought for digitally by cyber soldiers will attack friends and foes alike, they will malfunction dong the same thing or even simply going rogue on an almost AI like level, a true terror to consider while knowing of their accuracy that only a machine can provide. Debilitating weapons of all kinds will be used manipulating light, sound, microwave frequencies and much more. They will be amplified to the point that enemy combatants are permanently disable if they're unfortunate enough to survive.
There will be psychological warfare on a scale never before seen, including but not limited to induced hallucinations, a reckless disregard for rules of engagement, ruthless slaughter of civilians in the most brutal ways imaginable and their remains left for all to see, not including damage done as consequence of bio weapons. The weapons used will terraform the planet in both shape and inhabitability in which areas become impossible to survive for even a short while. Some areas so bad and changed that there's no oxygen of any kind to be detected let alone breathe. The consequential lack of resources around the world and polluted ecosystems will end up destroying the worlds population every single day for decades.
Truly the unfortunate souls that survive WW3 will wish they hadn't.