The master-blasters that win the big 3 Gun matches typically use low power variable Leupolds (like 1.5-5, etc.) They evaulate the course, pick their magnification, and hose away. And, the magnification can be changed very quickly on the run.
From shooting some 3 gun matches with my compact ACOG, here are some observerations:
1.) Hold over is very hard to do accurately with a triangle or dot. This is particularly importand to consider, as most 3 gun rifle matches have targets at various distances.
2.) Fast target acquisition is key. Low magnification is great for this, as most of the targets are at least 40 yards away. A red dot may be fast up close, but I think they slow down for long range shots.
If I was going to redo my setup, and optimize it for 3 Gun, I would do this: 20" Flat top AR with a very effective comp (not a flash hider) and an ACOG TA01NSN. The TA01NSN has BDC built in, so it works very quickly**. The 4x is also pretty fast. But, I've already got a setup I use that works great for the majority of my competitions (close range CQB type stuff.)
**I know the NSN sight is optimized for a 14.5" barrel, but it is close enough for a 20" barrel to work good. My step Brother (finished 32nd at the 2002 World 3 Gun) uses the TA01NSN on his 20" AR.