Used Car Salesman

Used Car Salesman is a Halo 3 Achievement. It is awarded for destroying a vehicle with three enemies in it in either a ranked online playlist or in campaign. Unlocking this Achievement gives the player the Scout Helmet for Spartan Armor customization, it also awards the player five Gamerpoints. Note that Scarabs do not count as vehicles for the purpose of this achievement, as the enemies must actually occupy a "seat", rather than merely stand aboard the vehicle. It is represented by an eight-pointed yellow star on top of an eight-pointed red star. On top of this are three white stars.

Method 1
An easy method to get the achievement is to start a campaign mission where there are enemies on prowlers, then weaken the prowler with weapons fire or frag grenades, then destroy the vehicle with a rocket launcher, spartan laser, or a plasma/spike grenade stick.

Method 2
Alternatively, you can also destroy the front turret of a phantom during a campaign mission. This can cause a glitch which unlocks the achievement, as the game thinks the phantom is destroyed when only it's turret is.

Multi-player
The easiest way is to get it on a large level with Warthogs, Scorpions, or Prowlers, like. Grab the Rocket Launcher or the Spartan Laser. Wait for the other team to grab a Warthog and destroy it. It is best to do it on a CTF gametype, since their players tend to use team vehicles.

Internal

 * Halo 3 Achievements