Your biggest challenges will be the reloads and the trigger. You can get trigger work done by several good 'smiths, but they won't be quite a good as any of the "drop-in" AR trigger sets (Chip McCormick, JP, etc).
Even with a Tac Latch you will have to strip the mag out during a reload, and that will cost time.
Mag holders and the like aren't a big deal:
3 Gun GearTheir mag holders for the AR15 will likely work since they can be "formed" by storing a mag in them, but you may want to e-mail or call Mark to make sure.
I've thought of using my HKs (91 or 93) in 3-gun, but I think the only division it would be worthwhile in would be He Man. FALs and M1As don't have any real advantage there, and I would argue that the 91 would be better than an M1A for reliability reasons. I can't begin to tell you of how many shooters I have seen with unreliable M1As, and I don't know if that's inherent in that platform, or if it's something that those shooters are (or are not) doing. The FAL is about equal to the HK there. Tactical (Limited) and Open are dominated by the AR15 platform, primarily because of the 3rd party accessories available.