I think any solution to provide 3 round burst on the semi-auto selector position will require bypassing the trigger mechanism, since you'll need the power going through the MOSFET regardless of whether the physical trigger contacts are open or closed to continue cycling the rest of the burst. This limits you to more expensive/advanced wired in FETs.
The only simple drop in solution I know of will only be able to change your full auto to 3 round burst, since power is still going through the trigger contacts:
http://www.aegwizard.com/AEG_Burst_Wizard.html