For the second and third people he shot, I agree. The reason he shot the first guy is more murky.
If the first guy is inappropriate he doesn't have standing to claim self defense in the other cases.
I've heard the first person he shot in the head from across a street. If it was cops pursuing him to catch him and he shot them, would it not be murder?
Exactly. Once you have unclean hands you don't have the right to use force in self defense relevant to the situation in which you have unclean hands. Once he shot someone without reason any force he uses against those trying to stop him is automatically illegal.
Being 17, he was also illegally carrying that firearm.
Check local law on that. You need to be 18+ to buy a rifle/shotgun and 21+ to buy a handgun but that doesn't say you have to be of those ages to possess one.