After completing the Prologue, and liberating the first Outpost, Royal Cargo trucks will begin to appear throughout the world (look for the blue truck icon) and will be available to steal. When one appears you can either steal it and return it to the nearest Outpost, or destroy it and receive a smaller reward. Hijack 3 of these trucks and return them to unlock this achievement! To hijack a truck just shoot the driver or shoot the truck until the driver gets out, shoot him and drive off. Don't worry if you miss the truck, as the truck seems to come back to your position after about 30 seconds. Good luck!