Seriously cut the "He was just a kid!" crap. 15 is not a child, not really, a 15 year old is capable of seriously harming or killing any other individual, throw a firearm in the mix and you can go a lot younger.
I'm not going to rant about how shitty guns are, it's not due here but well...would be better without 'em...
Anyway, police seem to be justified, it's hard to tell sometimes, it really is. I did my officer training and honestly at a distance, under pressure it's damned near impossible to tell the difference between plastic toy and actual firearm, mostly go off how it's being held, if they're holding it in a threatening way and are putting yourself or others at actual risk then the call is there to be made and it's hard to judge someone for making it the wrong way.
In a country where access to firearms is so easy I would not be willing to take the risk. As for 3 shots, well it could be if the kid simply didn't drop, a lot of people can take a bullet and still pull a trigger, you should double tap for a kill and if they don't go down you fire a third round.
eta: The more I read the more it sounds like suicide by cop and/or pure insanity, I can't see how the police weren't justified here. He had multiple choices and actively threatened to murder people and was in a position to do so, then he raised his pistol...