This achievement involves killing three enemies at once (a triple kill) on ten seperate occasions (in the single player campaign).
There are two prime spots for doing this. The first is in Act 1, where you are breaking out of prision. There is a point at which Dom tells you to take cover as the Locust start forcing their way in through a metal door. As the door is blown off it's hinges, three grubs charge through the doorway. This is your chance to get a triple kill. If you accomplish this, simply run to the next stage and allow yourself to be killed. You will then respawn at the point where the three locust are about to break through the door, where you can repeat this procedure as many times as needed.
The second, and my personal favourite, is another point in Act 1, just after the point at which you cross a high bridge and see two dead COG soldiers next to a van and a large hole in the ground. Kim uses his code to open the door, and your squad have to fend off hourds of Wretches in a long hall.
Often these Wretches group together, and if you throw or tag an enemy at the right time, you can blow three (or more) of them up at once. If you did this successfully, you can let the remaining Wretches kill you, at which point you will respawn just at the point before the Wretches emerge.