Three Gun is Great I have several good friends that compete in it regularly! I do not because of time constraints. And I do not need another addiction to weapons since I already have enough!
But I have been with them when they competed. The other competition that you may want to look into is the NRAs Rifle Competition in the service rifle area of competition with the AR-15 NM Rifles. They have local clubs all over the USA that participate in the Department of Civilian Marksmanship Competition Programs. The National Match Competition is held at Camp Perry Ohio the end of July and first of August each year. If you ever want to go see a shooting event it is one of the best. They also have vendor stores all up and down vendor row! Not tents! Actual Stores! Rock River - Bushmaster - Armalite - Springfield - Fulton Armory - Gun parts galore and accessories for the shooter!
And some great deals on everything from Stripped Receivers, Triggers, Complete Rifles including their best Match Service Rifles manufactured to NRA / DCM Specifications for competition. There are Service matches at the individual clubs throughout the US and at Camp Perry which use the AR-15 National Match Rifles from those manufacturers. And also the clubs have M-1 Garand rifle comp and others. Something you might want to check out! Just for example if you want to take a look at on of the match rifles they use for competition for your information, go to Rock River Arms and for example look at the Model AR1286 or AR1285 Both are AR-15 National Match Models.One has a removable Rear Sight assembly and the other is fixed. Rock River sells about 200 of these NM Rifles there every year. Like I said Armalite and Bushmaster also makes match rifles. I prefer the Rock River it is one fine competition DCM Rifle. I personally have a Model AR1285. If you want to do some serious competition that is it. You could compete at the local level all year long and even go to Camp Perry for the National Matches is you chose to do so! They also have the M-1 Garand service rifle matches at Camp Perry and most clubs also. In the M-1 Garand service rifle match at Camp Perry however, they do not permit National Match Grade M-1 Garands to participate in the matches. It must be of service rifle grade.
Good Luck & Good Shooting!