You don't have to actually shoot 3 people during the eQM, just have 3 people die. You can do this much easier by shooting mines or gas bottles during eQM.
There's a great spot for it on the 4th floor of the bell tower. There's 3 or 4 enemies taking cover behind some crates and an orange gas bottle in front of them, just eQM accross and shoot the gas bottle to kill them all and get the achievement.