so the problem with finding this chip is that it has to work by using the added button as the RF trigger, and not on the trigger. Most people are designing these chips with RF on the trigger, thus the chip has no idea when the trigger is pulled, thus making this hard to accomplish on the trigger(not impossible)
The whole idea is silly tho, cuz all you have to do is pull your trigger in short bursts with rapid fire on, and one of the usually 3 modes will give you desired effect. I don't know anyone who would really want 3 round burst, cuz as you get used to RF, you never really pull trigger and let it ride, you are constantly tapping the trigger for bursts anyway.
If you really want the 3 round bursts, and money is no object, Let KingMike know, and I am positive his programmer can get something going for you in short order
If you are looking to save ammo on a full auto weapon just use one of the slower RF speeds or just tap the trigger like you would if bullets were flying past you head in the desert.