Just trying to flesh out was x2i4eva said.
I got this on the first level in the game (Sierra 117) towards the end. The trick is to save your plasma grenades for when the Covenant dropships role in. Throw you grenades at the turret at the front of the ship, at the bottom. Although the vehicle won't be destroyed, you will still unlock the achievement.
Or you can try playing a ranked playlist, hope you see three guys in a Warthog and kill 'em all while they are in the vehicle.