eQM is the slow-mo (using Y between cover) mode. You have limited time, ammo, and cursor movement while in eQM. So the key is to attempt this achievement when there is a tight cluster of enemies, but unlike "Glad I wasn't there" the enemies do not have to be physically close together. Both of these achievements are easiest in corridor and tunnel stages.
The shoot 3 guys method. ;)
Using the shoulder width of your character is a good guide for how close the 3 enemies should be. It's fine if they are running, as once you start the eQM all motion will stop.
- be in cover, with 2 bullets of adrenaline
- reload your weapon (I recommend the pistols for precision)
- try to line up your cursor near the center of the enemy cluster
- start the eQM
- try to fire no more than 2 shots at the heads of all 3 enemies, you will definitely have time for 6 shots, and maybe more
- the reticule will turn from red to grey if your target dies, but don't wait for it before moving on to the next target!
- if you have time left, sweep back to see if the reticule turns red and keep shooting