One shot, center mass will at least phase an attacker, even one wearing body armor. This will free you up for a second or two to address the other attackers if they exist. After at least temporarily disabling all attackers, you can return to them for follow up shots as necessary. After the first shot, they will be moving much more slowly, and you've got a higher chance of placing a second shot in a more vital or less defended area. There are few animals, people included, in this world that won't just fold when hit with a bullet. Anyone who hunts or has been in combat, knows this to be true.
The 'two in the chest and one in the head' drill is really only necessary when running quickly through a large group of people where you won't have time to place follow up shots. It takes a considerable amount of training to perfect and honestly, would be hard to explain to the DA, as it is done to purposely kill an individual, rather than incapacitate an attacker. You may be tried for murder in a self defense case and that needs to be in the back of your mind when training for self defense situations.