It's Brianenos.com. Well-known 3 gunners Todd Salmon, Kurt Miller, Bruce Piatt, and even Mike Voigt post there.
You need to figure out which class you want to shoot. You also need to look at the rules of the various matches that you intend to shoot. There are 4 major 3 gun associations: USPSA/IPSC, WC3Gun (old SOF), International Multigun Assoc. (Mystery Mountain 3 gun and Rocky Mtn. 3Gun Assoc) and North American Tactical 3 Gun (vikingtactics.com). Of course, they all have different rules usually in regards to sighting systems.
In my opinion, it's better to build a custom gun slowly rather than buy an AR and then start adding pieces. That way it's like Burger King, you can have it your way. Here is how mine is set up (by Accuracy Speaks):
1) Barrel: My main competition AR has an 18 inch medium weight Douglas barrel 1 in 9 twist. It will shoot everything from 40 grainers to 75s. You don't want a short barrel! They may be more handy but a full-length gas system makes for less recoil and less gun movement. I can't think of one truly competitive 3 gunner that uses a 16 inch barrel. You don't need a match grade barrel but it's nice. Don't get a twist slower than 1 in 9 as you may want to shoot heavier bullets.
2) Compensator. You need a comp. ARs don't have a lots of muzzleflip or recoil but you'd be surprised how much a good comp will help. Look at JP (Get the Cooley comp as it's legal in all matches whereas the fullsize one is not), Miculek, Benny Hill (triangleshootingsports.com) and Accuracy Speaks Jakenator.
3) Free floating handguard. I like Accuracy Speaks and Scott Medesha handguards but there are lots of good ones. You want a freefloating tube because sometimes you can use bipods and ching slings.
4) Slings. Have a ching sling and a vikingtactics.com tactical sling.
5) Triggers. You need a good trigger. For 3 gun work, most (but not all) prefer a single stage. Accuracy Speaks and JP are both good.
6) Sights. You'll want to be able to shoot both irons and scope. Flip-up front sights are cool but they do have some zero problems. Get a gas block that has a weaver rail (armalite or JP) and a detachable Armalite front sight. Put a skinny front post in it (.030 to .050). Get a detachable handguard A2 style sight and put a same plane apeture in it (Ashley Outdoor or Bob Jones).
6a Scopes. This is tough as the rules diverge. I think the low-powered variable is the best allaround sight (Leupold 1-4X, 1.5-5X or the Zeiss 1-4X). They are what wins in open class but they're not legal in tactical scope class at Mystery Mountain or WC3gun where you must use a military-issued scope off the approved list. Low powered variables are legal at North American Tac 3 gun and Rocky Mountain 3 gun.
The best sights off the list are the ACOG TA11 and the EoTech. The ACOG is a lot better for the medium to far shots and the EOTech is better for the closer stuff. I'd go with the ACOG unless the matches you're shooting are heavily focused toward the close stuff. Yes, an Eotech can make 400 yards shots but they are not really competitive once you get past 200.
7) Stocks. I have an A1 stock with an adjustable buttplate which makes it A2 length. Nice but not at all necessary.
TriangleShootingSports and Clark Custom Guns both make a competition ready AR that is relatively afforadable. JPs are a step in both price and fit and finish but not necessarily competitiveness.
Good luck.