Nobody has made a 3rd burst FRT. While it is theoretically possible, it would be difficult to design and build something reliable. Heck it might be more difficult to make it for an FRT since the FRT is resetting the trigger every time whereas the FA doesn't. You might be better off learning to use the trigger and get consistent short bursts. Even the 3rd FA burst for the M16a2 was pretty bad. Eugene Stoner said that 3rd burst was the worst option. You would be better using a 5rd burst instead. That gives you time to react to the muzzle climb and pull back on target. Usually a 3rd burst will string vertically and by the time you've figured out the recoil, you have to pull the trigger again. Shooting 5 rounds with FA or FRT should be pretty easy, especially with a heavy spring and lower bolt velocity.