What a world war really is many countries allied with eachother who are fighting against many other countries that are fighting against eachother. The war zones will not only be spread across countries, but across continents.
To answer the OP's question, no nukes will not be used. The reason for this is because a world war will never break out because of nukes. Countries are too afraid to go to war with eachother directly because of how close to nuclear destruction we are even without war.