3gun teaches people to be shooters when in reality you need to be a gunfighter, or better yet, just a fighter. They are teaching you to maximize firepower, when in reality many fights and conditions aren't about doing that. They aren't teaching you recon, or how to take best cover, nothing about strategizing the actual fight in front of a man in battle, just attacking and shooting in and of itself purely. You aren't taking a position, looking for enemies and assessing the situation, you just plow through hard straight ahead and attack, attack, attack! The focus is on score, not survival, not victory. If you are also not using your mainline weapons and equipment its not very useful as real training because you aren't even practicing with what you have. Also no squad, yes, all those points are valid.
There is no "crack head with a knife ten feet behind you while you carry groceries to your car in the parkinglot" scenario for self defense purposes. There is no "coordinate with your squad and platoon and heavy weapons section while you maneavuer to take an enemy position through careful coordinated executions and mass almost half blind fire to suppress an enemy" either. I don't think they have any situations where the enemy cardboard cut outs come and assault you or your position. There's no bonus points for knowing when a position is untenable and retreating out of it fighting.
Its good but its not perfect and if you rely on 3gun or other competitions entirely you will get the wrong lessons that will have to be untaught. "Don't get hit" is the old samurai value to be learned, and 3gun teaches none of that.