Once the firebomb is available (Rank 4), select the grenade by holding and pressing left on your left analog stick to highlight it when you release . The firebomb grenade launcher doesn't work for this achievement and sometimes the soldiers count, but not every time like they should. I think "enemies" should've said "aliens" or "Xenos" to make this easier. In any case, there's multiple locations in the campaign where throwing the firebomb will net you the achievement. Personally I used mission 1 (Distress) when O'Neil needs to be defended while cutting open a door. Restart checkpoint gives you multiple attempts to make sure at least 3 catch fire from one grenade toss.