I have the same fear, but like everyone said worrying about it isn't going to prevent it, it's just causing unneeded stress. It's out of our hands, whatever happens will happen.
It's hard to not obsess on thoughts like that, I know I've had a lot of other weird thoughts that I constantly worried about until I realized that it was all futile. Try and get out and enjoy life, clear your head, and stay away from Google. Searching the net has made me worse sometimes as far as fear goes.
I got a lot from anti-anxiety drugs. It didn't take a very high dose to remove the phobia I was subject to, and it eased a lot of free-floating anxiety. It's not like worrying is going to prevent WW3, it's either going to come or it is not going to come, so you might as well have an OK quality of life in the meantime (or for your whole life, if WW3 doesn't come). You might see a doctor.
I understand how difficult it can be when things like this take over our thoughts. We don't know if or when things like war are going to happen but worrying about them is going to cause needless anxiety. It is hard because something like war is out of our control.
You are so young and you have a long life ahead of you. Try and focus on positive things and enjoy your life.
I wish you lots of luck. Remember think positive thoughts!!!
its normal to feel that way look at me for a long time i thought the end of the world was coming. couldnt even sleep at night now im stuck on my health thinking something bad is going to happen to just me. the only thing i can say is just focus on good positive things around you and pray and leave it in Gods hands .i have been told if you can practice and keep your mind on good things that are around you and that you enjoy the the bad thoughts will fade............