You'll need to unlock all of launches 6 tiers then choose multi-launch to the right of the 6th tier. It will cost 8* points (which if you have all DLC will require the last 5 hidden points in the AWE DLC and 3 from watching the cut-scene explaining Hartman) or a reset of your abilities to unlock (30,000).
Once purchased use your launch skill (default RB) and pick up 2 additional pieces (default LT). Then with 3 enemies or more look at 3 of them for lock on and fire away. Once all 3 are hit the achievement should unlock.
Fixed value from 6 to 8