Converting an AR-15 to 3-round burst involves the following:
First, get a Class II license.
Second, mill the section of the lower out that projects a little (on the right side)
Third, locate and drill a hole for the auto-sear.
Fourth, replace the trigger, selector, hammer, sear, and sear spring with the appropriate parts, ratchets, cogs, gears, doohickies, and other delicate crap you'll need. You can get them from Bushmaster or any other AR-15 manufacturer with your tax stamp.
Finally, install the auto sear and M16 bolt carrier. You're ready to rock, roll, and complain like nearly everybody else who uses the neutered 3-round burst mechanism.
If you REALLY want a 3-round burst, the HK and Ruger mechanisms are much better. I prefer a full-auto mechanism and some fire discipline.