Don't get so caught up in the doomsday aspect of World War 3. Theoretically World War 3 is a violent war where nukes obliterate the entire planet. In practicality World War 3 would be a heightened political tension over various societal issues. We don't live in a world where WW3 is possible anymore. The cold war made that very clear. We'd probably fight proxy wars (like Vietnam) and it'd go on for decades.
The best thing you can do to survive World War III? Have good savings and don't succumb to the war effort. WW3 would probably put us in another depression so that's something to look out for.