The weapons squad has the three man gun teams and two guys assigned as javelin gunners. If needed, the ammo bearers for the guns were flexed to them to carry an additional round.
Glad to hear the Carl G has been adopted.
Outside of the manual, I never saw an assigned 3 man gun crew. If I ever got a ammo bearer is was mission specific, sliced from a rifle squad, or maybe one of the Dragon gunners if the rifle squads were full. But, all my time was Airborne of Light...so.....