I've shot eight or ten 3-gun matches, and have not bought any "trick" parts for either of my long guns, which are not competition guns, anyway. I look at 3-gun as way to become more competent with the weapons that I own, rather than a reason to buy specialized gear.
A larger-capacity shotgun would be good; most people load from a belt, but if you want to win, you'll need something faster. I can load 8+1, but the last match required the gun start unloaded on most stages, so the mag capacity didn't matter as much as usual.
I think most of the guys in my area are using a Blade-Tech mag pouch, but I don't know; don't think they make 'em for M1 Rifle or Carbine!