C.R.Sam, you bring up a good point, but I think some clarification is needed.
Statistically, multiple bad guys at varied distances armed with guns are not equal threats to you. While they may all be able to kill you from their location, it does not mean they they are all an equal threat to you. The greater the distance away, the less likely they are to be accurate. As the saying goes, proximity negates skill. I would rather gunfight with a fully capable person with a handgun at 100 yards than a legally blind person at 3 feet. That is an extreme hypothetical example and not realistic. However, with greater distance, it becomes much easier to miss the intended target. For handguns and rifles, that is a fact.
As far as shooting everyone once and then deciding on who needs to be shot a second time, versus shooting and evaluating how your shot performs, both have advantages and disadvantages. Shooting everyone once means potentially inflicting painful wounds to potentially incapacitating each person. If there are three bad guys, that means you will need at least three sight pictures and two transitions between targets, combined with the fact that after the first shot goes, your targets likely will not remain stationary, so you will have to be trying to transition while tracking and obtaining a sight picture. That ends up with a lot of down time between shots.
Let's say you shoot each one time because that is the good way to do it. Let's say the first threat you shot was the most dangerous to you as that would be the good way to do it. Let's say that shot didn't do anything to the greatest threat to you and now you are wasting time trying to track and shooting other targets when the one closest to you, most dangerous to you, has not been taken out of the game. Targets 2 and 3, by being more distant and likely trying to flee are not posing nearly the threat that target 1 poses, target 1 now being the really pissed off bad guy who was the greatest threat to you and still remains the greatest threat to you, only you have your gun pointed in a direction and are trying to find threats who may not actually be threats to you anymore.
Here, the key is that Target 1 is a threat and if you fail to nullify that threat, transitioning to Targets 2 and 3 isn't going to do you any good because Target 1 will be shooting you while you try to shoot Targets 2 and 3. Bummer, no?
The advantage of focussing on Target 1 is that you want to nullify the greatest threat to you and you selected Target 1 because he was the greatest threat. You know he is a threat, but once you start shooting, you don't know if Targets 2 and 3 will remain threats or not. So, you deal with the most pressing matter first and Targets 2 and 3 potentially will remove themselves from the battle or be ineffective while trying to egress. That is a best case scenario, of course, but it happens in real life. Honor amongst thieves and watching each other's 6 isn't terribly common. Of course if they are a trained hit squad come to kill you, then you are just screwed.
The disadvantage of fixating on Target 1 is that you can lose contact with the situation as it is developing, plus you can unload all your ammo into one target in a very brief amount of time. So, you can end up with a singular and very dead primary threat, and be killed by the secondary threats that you don't even realize are there. Fixation is a very real problem that can coincide with tunnel vision.
Sweeping one's head around to check for additional threats will help reduce the effects or nullify tunnel vision, a good thin. So, by being forced to transition to shoot each target may be advantageous to not getting caught in a tunnel vision mode, but once again, if you fail to nullify Target 1, then you end up being killed with normal, not tunnel vision. Being killed with or without tunnel vision does not matter. Dead is dead.
There really are no simple strategies or tactics. Gun fights are not static situations, but dynamic even if the shooters are static themselves in that they remain stationary. People around the shooters will be moving and as ammo and shooters go down, the situation changes.
Part of what makes situations dynamic is movement. Stationary and out in the open is bad, very bad in terms of potentially getting shot, but gives you the best shooting perspective from being fully upright (hence seeing more), stable shooting platform, and ability to pivot in place and cover a large arc. Moving while out in the open is much better from a point of trying not to get shot. Moving targets are harder to hit, but it also makes it harder to shoot well. Statinary behind cover may be good for defense, but you are limited in the range of motions to fire and your actions can become predictable. Plus, you can end up trapped.
Nothing is truly simple and nothing is fixed in stone.