I found doing this on level 14 on Desert Strike to be the easiest spot. There are three enemies here, conveniently lined up one after another.
Simply lob grenades at the enemies. Hit in between two of them for maximum effect, and simply kill off the last. I'm not sure if killing the enemies by blowing up the bridge from below and dropping them off the level counts as kills, but I didn't risk it. I only used my grenades to kill them.
If you have completed Lost City, you have been rewarded with the Detonator, which makes this a stroll in the park. Simply throw a grenade directly over their heads, then press the detonator to (most likely) blow up all three enemies in one go.
If you save this achievement for last, it shouldn't take long before you unlock it. I probably spent about 4 minutes doing this over and over.