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).
YMMV