The Metalstorm 3GL is an electronically fired 3 Round (!!!) 40mm low pressure grenade launcher. The rounds are stacked in a single unit at reloading, and can then be fired front to back in quick succession.
Arma 3 3GL's are single shot weapons just like the Opfor EGLM. This needs to be adressed: no point in using 3GL's when not making them live up to their name.
Source 1: Video demonstrating 3GL prototype on M16 (In AddInf below)
Source 2: Wikipedia article detailing the Metalstorm systems