There are three Avatar items you can unlock in Gears of War 3 for the Xbox 360.
Here are the Avatar items you can unlock, and what must be done to unlock them.
Horde T-Shirt – Get the “Welcome To Horde Mode” achievement by surviving the first ten waves of Horde Mode on any difficulty and any map
Locust Drone Mask – Get the “Welcome To Beast Mode” achievement by surviving all 12 waves of Beast Mode on any difficulty and any map
Marcus’ Doo-Rag – Get the Bronze Tour Of Duty medal by completing the campaign on the Casual difficulty