I am hardly a veteran 3-gun shooter, but I have shot in 1 match, and am planning on shooting in another this fall. It entails shooting shotgun, rifle and pistol, in different stages. Some stages will have you shooting only one weapon, and some will be set up for more than one. For example, you might have to hit several targets with a rifle, put the rifle down, run to another station, and start hitting targets with the pistol. On another stage, you might have to run a course and shoot a bunch of targets with the shotgun only. We had stationary targets, moving targets, pop-up targets, shoot/no-shoot, etc. As I recall, it's based on time. You are timed in clearing various stages, and the there are time penalties for things like "didn't neutralize target," and things like that. I didn't even come close to winning, but it was TONS of fun to shoot.
A gunfight is not the time to learn new skills.
If you ever have a real need for more than a couple of magazines, your problem is not a shortage of magazines. It's a shortage of people on your side of the argument. -- Art Eatman