Lt. Aldo Raine: You probably heard we ain't in the prisoner-takin' business; we in the killin' Nazi business. Business is a-boomin'.
Lt. Aldo Raine: The German will be sickened by us, the German will talk about us, and the German will fear us.
Lt. Aldo Raine: Each and every man under my command owes me one hundred Nazi scalps... And I want my scalps!
Chosen answer: It is explained shortly afterwards that a real German would hold up his thumb, index and middle fingers to indicate three. Since, the major already suspects the Captain of being a spy, holding up his index, middle and ring fingers to indicate three confirms this.
Xofer