Gran Premios are the vehicle races you find across the map. They have a black and white checkered race flag symbol. You do not have to complete 3 different races. You can complete the same race over and over. I completed one car race once, and a quad bike race twice in a row. Achievement unlocked.
The first time you complete a race you will get a vehicle part reward. Replaying the race will only give money as a reward.
You find new races by picking up the race posters across Yara. They will show up as an exclamation mark on your radar.