Everything comes to a stand still when there's a war so that wouldn't be different if a Third World War was to broke out. Bitcoin will suffer in such a circumstance just like any other thing or systems in place.
Have you ever watched war images? If there is a war, the bitcoins will not be in the same place. Your computer, your house, your car will not be in the same place.
Maybe you will not even be alive. Your family, you can lose everything you love.
Bitcoin is a money hidden in digital media. If you can not access your digital media, you can not access the bitcoin.