XCOM 2

Rangers can get the Reaper ability at the rank of Colonel. This lets you chain melee (sword) kills. The first strike is a guaranteed hit at normal damage. The second strike is based on normal percentages and deals lower damage. The third strike is yet lower damage.

Pick out your three victims. Have one of your other soldiers soften up the second one, if necessary. Switch to your Ranger, choose Reaper, get your two melee kills, and then switch to the shard gun for the third kill. This breaks your Reaper chain, of course, but it unlocks the achievement.

I'm sure there are other ways to unlock this achievement. The Faceoff ability of the Sharpshooter Captain comes to mind, but it seems like you'd need three heavily-damaged-but-not-dead targets who all happen to be out of cover at the same time for that to work, and that's pretty hard to set up. Doing it with Reaper should be a piece of cake.

EDIT with minor spoilers: at some point in the game, you will encounter an enemy called a gatekeeper. It has the power to revive multiple dead aliens and civilians as psi zombies. This gives you two ways of unlocking this achievement. The first is with the aforementioned Reaper skill. The psi zombies are weak, unarmored, and not in cover. They are no match for the Reaper. Just activate Reaper, then slash, slash, shoot. You might have good luck with the Faceoff ability here also. The other way is to kill the gatekeeper; all of its psi zombies will die when it dies.

EDIT: archer139 says, "The way I did it.. was throw an acid grenade.. and let the 3 Advent troopers melt.. all three died pretty fast." Acid grenades can be built in the Proving Ground as and Experimental Grenade project. You may get gas or incendiary grenades instead, however. You can research Experimental Grendae again if you want to keep trying for acid.

Now Am I Become Death26 (15)

I must say the other solutions are very good. I just wanted to add to them with a video of how I unlocked this achievement, using the "Sharpshooter". I hope you enjoy the video, till next time stay frosty!!

This is definitely not the best option, but if you have one of your Psi troopers use Void Rift (as long as they have Rupture trained) on three enemies it's possible to kill all three in one shot. In the same vein, you can also do this in the final mission with the Commander Avatar - his Dimension Rift ability has a large area of effect and can easily do enough damage to kill a group of Mutons, Vipers, Sectoids or Faceless if they remain grouped up.

As an alternative for those with the "Shen's Last Gift" DLC, the Overdrive ability of SPARK units gives them an extra action and any shots taken won't end the turn.

I got via a Sniper with the Serial skill at Colonel. Target a bunch of low HP enemies with good chance to hit, and fire away. Especially useful if you bomb a couple of Codex and they split, leaving several 1-2 HP entities to mop up.

Easiest way is to get the Sharpshooter ability to fire your pistol at every enemy in line of sight and wade into a group of troopers/cannon fodder.