The saiga 12 shotguns are very popular for 3 gun matches. I have several friends with them, and after a little work on the gas ports, it shoots pretty well. The biggest advantage of the saiga for 3 gun is the fact that since its magazine fed, reloads are very quick.
However, personally, I am waiting for the competition version of the mossberg 930 spx to be released. Benny Hill is testing them for mossberg right now. He says its the best out of the box shotgun in the $700 range he has ever seen!!