If a world war broke out it wouldn't be between the west and Islamic states. The Islamic states would nor wish to wage a war on the West or have even a chance of winning. Only a very small minority of muslims make up terroists, and these terroists rather than achieving long term goals are really wanting publicity for their cause, and perhaps reduced western influence. If a war was to break out, these terroists would have a face (i.e. they would have to fight with the west directly) in which they stand no chance. While many Islamic countries may be slightly annoyed by western influence and exploitation of their resources, other countinents have also suffered the same fate, e.g. Africa, South America. So no, I doubt that a third world war would break out just to fight Islam.
If there were to be a third world war it would be between developed western countries fighting for resources, just like in the two previous world wars.
But let's say this war had erupted where the islamic states declared a "holy war", I would not join.