Killing 3 enemies at one time will require a grenade, Boom Shot, Hammer of Dawn, or possibly a well-aimed sniper shot. The easiest way to do this is to do the following:
Start the game's campaign on Casual. In the prison, when you reach the door that the locust start to cut down to get through, hide behind the barrier closest to the door. Right before they break through throw a grenade. Time it and position it so it kills all 3 of them. If it doesn't, let the ones who lived kill you. If it does, run outside and let those Locust kill you. If you try to reload the last checkpoint through the Pause Menu instead of getting killed, it won't count. Repeat this 9 times to get this achievement.
Please note that you cannot reload the checkpoint as the video shows. If you do, the kills will not count. Instead, run outside and let the other locust kill you.