This can be the first achievement you unlock in the game, and is done on the training mission
S.S.D.D..
As soon as you complete the basic training on how to aim, shoot, and throw grenades, you will be allowed to leave towards The Pit. Instead, turn right as you exit, and follow the sandbags to find a
Barrett .50 cal Sniper Rifle.
Clown 1: From the sniper rifle's location, look into the field and spot a building. Just to the right of it, on top of some rocks find a scarecrow clown, and shoot it.
Clown 2: Head to the top of the watchtower, not far from where we started. Once on top of the watchtower, look across the airport runway to the closest hangar building. On the top corner, find another clown.
Clown 3: Head towards The Pit (main objective), but instead, go inside the small structure at the far end of the stage. Look through the window hole to the right, and spot a construction vehicle next to some tanks. Inside is the 3rd and final clown.
Upon hitting all 3 clowns, the achievement will unlock. Restart the checkpoint if you miss a shot.