It turns out EMBAR executions count towards this achievement.
Play casual coop vs AI and down enemies however you want (lancer is probably easiest since the AI drop ammo for it). Hold
while standing over their body with the EMBAR equipped to perform the EMBAR execution. Rinse and repeat to obtain 3 kills "in one clip", no need to worry about misses or ammo limitations.
Executions work for some of the other weapon achievements too like the Torque Bow.
You can even perform all 30 kills in a row (credit to Xx JaY2SiKk xX & Agnt Washintub)
EMBAR appears on the following maps: