If you're shooting 3-gun, you're probably going to want fast target acquisition, so I'd suggest either an Aimpoint or EOTech. Although the ACOG is nice, and you can work up to being able to bring it on target fairly quickly, for the ranges in a 3-gun match, I don't know that you're going to need the ACOG (although I could be wrong, it will also depend on the stage construction).
I've shot 3 gun with an Aimpoint Comp ML2 on my RRA M-Faux. In my case, the optic wasn't the best, because the stages had a couple shots at 200 yards on 9" circle steel plates, which is very tough to do unsupported with a 4MOA dot on a carbine. With my setup, there was basically no room for error. Most of the competitors used a variable scope and did point shooting for the closer targets. I agree with 13Fox, it depends on how they set up your stages.
+1 on the variable scope from what I have heard. I do not shoot 3 gun but have read forums where people do and they like the variable optics like the Millett DMS, IOR, S&B, etc. Get something from 1-4 or in that area.
The best thing for three gun would be an EoTech with their FTS (Flip To Side) mount it stays on the rifle therefore keeping you out of Open division but giving you the dexterity to switch between 100-300 yd target to 25 ft in one snap! Spring loaded too. But.......... VERY expensive. There my 2 cents
If you could afford it, one of the ACOG TA31 models with a mini red dot mounted on top. These have become my favorite setup. Second choice would be an Aimpoint or EOTech with a flip to the side magnifier that you could swing into place for the longer shots.