Halo 3 - Used Car Salesman Achievement

Destroy a vehicle that has three enemies in it in a ranked playlist or in campaign.

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During the campaign you can get this by blowing up the area of a Covenant dropship where the turret is and for some reason it awards this achievement. Some people have got this by blowing up the front turret, some have got it by blowing the the side turret that is manned by an enemy, so try both. There are plenty of these ships in the campaign, any of them will work.

At the very first mission you can get this, just shoot at the turret or if you pick up a sticky grenade stick it which is much easier.