This achievement can be unlocked on any difficulty and on any level, assuming that there are three enemy units in the area.
To get this achievement, you must destroy three (3) enemy units with a single hand grenade. Odds are you will unlock this through normal gameplay as you work towards completion of other achievements. But here are some tips if you are struggling.
1.) Play on a lower difficulty: Lower difficulty means that the enemies have less health and will die easier from your grenade blast
2.) Upgrade your fragmentation grenades: I don't use the frag grenades all that often, so any time I picked up more grenades, I was working towards upgrading capacity and more importantly, damage. If you struggle to get three kills with one grenade, than try upgrading your frags first.
3.) Use an EMP: Its true that you can't outright kill an enemy with an EMP. But the EMP blast will stop any enemies from moving. So if you see a group of at least three enemies huddled together, hit them with an EMP grenade first, and then toss your frag.
4.) Pick your enemy: The red foot soldier robots, or the cannon fodder enemies, are the weakest in the game and they normally show up in large groups. Try to focus on these enemies to get the achievement more easily.
I have included a video to show how I unlocked the achievement, hopefully it will help clear up any other questions.
If you down vote this guide, please leave a comment as to why you did so. This will help me improve this guide along with any future guides I make. Good luck, and happy hunting!