Let me address a couple of things in your post because I'm really not sure where you get your information from.
"Even a police officer yells to put down the gun many times before shooting." Not necessarily. I work in law enforcement. If possible, you give warnings. However, if someone is pointing a firearm at me or a third party, they've bought a visit with a .40 caliber round; no warning required.
"THREE shots, come on? couldnt aim for the leg?" I can't speak for the average citizen but we're trained to shoot at center mass and to do so in 2 shot bursts to which will maximize your chances of neutralizing the threat. You never, ever shoot for the extremeties except in unusual situations (eg. someone wearing body armor). Most shooting situations are stressful and chaotic. Even when shooting for the largest area the body, most people will miss the majority of their shots. Factor in adrenaline and the fact that no one is simply going to stand still and let you choose what body part to shoot and you can see why. If we have to shoot we're doing so to incapacitate and stop the threat, not necessarily to kill. You certainly won't stop someone's ability to use a gun by shooting them in the leg.
I would suggest that you do a little research before you make statements like these in public.
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