Ruiner

Instead of being an idiot like me and trying to use the barrels for this achievement, go for unlocking the "Frag Thrower" skill (the sixth skill on the right). I completed this using just the initial unlock on the skill.

Throughout the game you'll run into situations where you're encountering numerous enemies at the same time (especially in "The Epilogue"). All you need to do is get three enemies within the "bubble" your frag drop shows and as long as all three of them die you should unlock this achievement.

Hope this guide helped! If you have any questions or concerns, lemme know!

In Mother's Womb towards the end of the game, you'll get locked in a red walled square area. Once you clear this, the next area has 3 of those corpse piles spawning enemies. Just wait until they gather around and use your basic grenade ability to kill at least 3 of them at once quite easily.