If WWIII happen, I think no currency will be main because we know bitcoin need internet to be useful and the country affected by war has no more uses in any different kind of currency so the not affected area will be benefit on the war happens. so basically, bitcoin will be main in other country that support cryptocurrency uses and they will benefit on this battle happens not the affected country.
Thinking that the internet will stop working during a world war is delusional. I don't believe there will ever be a large scale country world war, like we experienced it through WW1 and WW2, however we will have several proxy wars fought by para military units and groups, who are supported by a specific country without wearing their banners.
Society is exhausted of wars and no human would support or go to war by him/herself. And I truly hope that small scale countrys start to adopt a little bit in crypto currencies so we truly have some kind of escape area when the big governments (europa / usa) start to go after our money after the next halving.