After unlocking Trevor by completing the "Mr. Philips" story mission, call the other two playable characters with your in-game cell phone and pick them up with a four door car. Go to the Los Santos International Airport at the southern end of the map. Entering the airfield will give you an instant 3-star wanted level, so drive from one end of the airport to the other for three minutes while making sure not to lose the cops to get the "Three Man Army" achievement. To make this less difficult, upgrade your car with the bullet proof tires from the car repair shop. Note: It is possible to miss this achievement depending on the game ending you obtain, so make sure to get it before the last five story missions.