Or....SEMI AUTO!!! Its what real soldiers use!!!
I've only in my entire military life(2 years!) used an M16A2/A4 in burst for one iteration of fire(about 20 rounds).
I agree with you here. There is a big difference in ballistics between a .223 FMJ and a .20g plastic BB. Recoil, distance, speed, trajectory; to name a few. Not to mention, you're looking at holding 30 rnds with an AR, as opposed to 400 with airsoft. At a 3 rnd burst, 10 shots vs 40 is a big difference.
IMO, a 3 rnd burst would be nostalgic, and interesting, but I don't see the functional purpose behind it. As for learning to time/count your full auto bursts, put a phrase to it. Sound off when you fire, and you'll be close.
example: For the M60 and the .50, we used the phrase 'Die Mother F*er Die'. That was a 10 round burst.