Well, as funny as it is, that's very true. I don't think that a World War 3 would be too kind on people considering the weapons we have available worldwide at the moment, a true departure from the other two wars. But if you still want to invest in such times you need just look at the past two world wars and see what was sold/bought the most.
Nuclear war wouldn't help anyone. All stocks would crash. Nations would fall. This would not be a profitable war.
If WWIII happened I seriously doubt that there would be much action on the stock market. With all of the nuclear bombs around the world it wouldn't take long for one of the smaller countries with a bomb to set it off. After that it would be a chain reaction with everyone else.
Well assuming we don't all die from a nuclear annihilation I wouldn't invest anything in the stock market. There would be no point, the stock marketing would crash. Trying to predict something like that is almost near impossible. If I had to guess I think military stocks would probably come out on top.
If there was a third war then the prices of basic commodities like food and clothing would spike. If you want swift returns investng in a company which sells either foodstuff or clothing would be a wise decision. I think the value of gold would plunge though.
I recently downloaded and installed Neocron 2 (cyberpunk MMO) and wondered about the future in a similar way. I'm personally not bothered with a nuclear war and know that I will be long gone before anything like Fallout 3 could happen. What would be the point in investing when the satellites stop working? My guess is that people would go back to trading and bartering scavenged goods.
If technology collapsed we would go back to the simple life and possibly we would be able to build a society that is more fair than our current one.
Stocks-wise, I'd say pharmaceutical companies. But lets be practical, who would want to invest money at this time? I bet most of us will be thinking about safety and will be spending on security measures instead of thinking about business.
In my opinion third world war wouldn't be my top concern. However, I believe it would affect stock markets negatively and by that time there would be only a handful of risers. Although it would be good time to invest if the war wouldn't last too long.