I watched a 3 gun match today and found it most interesting. Interesting enough I think I would like to try it. I feel comfortable with my auto pistols and shotgun but would like some input on setting up my AR. I'm getting a little older and my vision is not getting any better so I was thinking maybe a Flattop upper with some sort of optics? Which barrel length would be most advantageous? Brake or no Brake? Any suggestions or recomendations?
Thanks, Rabon...

Yes, the flattop upper is a good way to go.  The popular thing right now seems to be some type of dot optic which will co-witness with the back up iron sights. This also allows you to verify your zero has not changed without firing a shot. Comps are good as they help the rifle shoot flat.
A lot depends on the distace you will have to shoot. Here in the SE our longest shots are normally 100 yards and dots and 16" guns can do well. Head out west and you start seeing 200 and 300 yard shots and then you really need the extra velocity of the 20 and 24" inch rifles and low power variable scopes become popular.
Do some searches in the competition forum for more info.

14.5" and 16" barrels seem to be the most popular, because they're smaller, lighter, and more maneuverable, but a 20" barrel will work just fine. Ditto the red dot sight recommendation. They're the fastest at target acquisition, and I've used a Aimpoint Comp on steel silhouettes out to 400 yards with a 14.5" M4 barrel. Learn the holdover for your rifle/ammo combination and shooting long range will be a cinch. A muzzle brake reduces muzzle rise, helps you get back on target faster, potentially saving you fractions of a second, and every second counts. You might want to consider a competition single stage trigger too. I use a JP single stage. It's crisp and light and the reset is short. HTH

Thanks for your help. I have a Bushmaster Dissipator with the A2 upper and a complete DPMS lower waiting for an upper. I agree, I will need a better trigger. I am going to use a Flat top and I am leaning towards the 20" barrel as I have the 16" Dissipator. In the Match I watched there were 4 very small targets(I would guess them to have been about 6"x6") engaged at about 200 meters after a pretty good run, based on that stage of the course I felt a good fixed low power scope would be the answer but from your input I will look at the Red Dot type sights and learn more about them.
Thank you and have a nice evening, Rabon...