This achievement requires that you kill three enemies with a single frag grenade. While the official achievement description states that you only need to "damage" three enemies, you do in fact need to fully kill all three from my experience (this is stated in the in-game achievement list as well). Presumably this can be done in multiplayer, but it is easy enough to do in campaign.
Load up Mission 2: Unified Terrorism and make sure you have several grenades. If you do not have any, purchase some through the in-game store which is accessed by tapping the upward pointing arrow next to the radar map in the top left corner of the screen (while in a mission). The cost is 3 grenades for 1,500 credits (a small sum, really). Progress through the mission until you are tasked with activating the second of two computer panels. Just after you activate this panel, you will have to defend your partner Saunders as he is crouched over to the right. As soon as you touch the panel, move the right to the platform overlooking Saunders. In the back of the room, around six enemies will spawn and begin to run towards Saunders, shooting at him. Throw a grenade basically at or just slightly behind Saunders. It will roll, and the enemies will scurry forward to escape the grenade. Now, cook another grenade (hold the grenade button briefly before releasing it) and chuck it closer than the first one, into the large mob of enemies. They will not have as much time to escape because it was cooked and if things go your way, it will explode killing at least three of them.
This will probably take several tries as the enemies spawn and behave slightly differently each time. Just keep at it as a bit of luck is involved. If you chuck your two grenades and don't get the achievement, just let the enemies kill Saunders. This will fail the mission and you will begin at the checkpoint right before pushing the computer panel. Even better, this also seems to restore your grenades that you used about 80% of the time, so you wont have to keep purchasing them that quickly.