There arn't many airsoft guns I know that fire in 3-round burst. Your options are pretty much limited to Systema rifles ($1500), and Baretta M93r's from companies like KSC. The Tokyo Marui Type 89 and 89F (rifles used by the Japanese Defense Force) have 3-round burst, but they arn't worth it. Magazines are harder to come by. It's a TM so it is inferior quality anyways. So it looks like you're pretty much out of luck. Of course, you can just train yoursaelf to let go of the trigger after 3 rounds on full-auto, but a 5-6 round burst works better in airsoft, and is easier to pull off on full auto.
"The deadliest weapon in the world is a Marine and his rifle."-Gunny
"What is your major malfunction?"-Gunny
"God was here before the Marines, so you can give your heart to Jesus. But your *** belongs to the Corps!" -Gunny
"They've got us surrounded again! Poor bastards..."
"The 9mm is a .45 set on 'Stun'"-Unknown
Captain of Section Eight