If world war 3 happen, then we should think about ourselves . are humans survive the ww3 or not? bitcoin and the other thing will become letter, frist thing are ourselves . it there were no any human exist then how could bitcoin and Internet exist?
Exactly, when WW 3 happens Bitcoin and Gold will be useless, our currency will be essential stuff to keep us alive like food and water, and maybe guns too 
You know what else is scarce ?
Ammo
Maybe we will even use Ammo as currency just like Metro series
I think peoples will survive and like what you told is correct gold and bitcoins will become more precious and the value will be cannot imagine, but then also same thing for survive you need food so everything is needed but for that currency needed and the supply also wanted.