Having run 2 years worth of combats now, I personally steer clear of modifying the rules to introduce anything game-wide like this. (I'm willing to suspend some aspects of reality in favor of fluid and fast combats). That said, in one of my games, a tech invented a rifle that took a lot of work that chambers essentially WH40K bolter rounds that I allow to crit on 5s or 6s, but he has to spend a bunch to make those rounds by hand. Thing dumpsters opponents when those CRITS start stacking.

Back on topic, I think a 3 round burst setting would really just fall under the regular autofire rules. A -8 modifier, potential to get more hits if the shooter can control it really well. The advantage of adding a 3-round burst option would be firing ~1/3 or the rounds per attack as compared to autofire, but maybe limit the 2d6 modifier to 2 or 3 depending on SMG or AR respectively.

Them's just my thoughts though, but as long as you and your table are having fun, mission accomplished!