The three robbers were a trio of men who killed Ackerman and his wife before kidnapping their daughter Mikasa Ackerman in the year 844.
The appearances of each robber varied. The leader was a stout man with a short beard and wore a wool cap. The axe-wielding robber appeared to be older, and was taller and thinner with short-cut, blond hair and a thin mustache. The robber who arrived late on the scene appeared to be the youngest, well-dressed and clean-shaven with short, brown hair.
In the year 844, when Mikasa Ackerman was nine years old and living with her parents, the robbers arrive at the Ackerman clan residence. Hearing a knock at the door, Mikasa's father opens it thinking the visitors are Dr. Yeager and his son Eren. When he does this, the leader among them stabs him, calmly walking into the house with his men. Mikasa's mother attempts to fight back against her would-be robbers, but one of them wielding an axe strikes her in the shoulder, killing her. He then approaches Mikasa, punching her in the face and knocking her unconscious.
Appearance: The appearances of each robber varied. The leader was a stout man with a short beard and wore a wool cap.
History: In the year 844, when Mikasa Ackerman was nine years old and living with her parents, the robbers arrive at the Ackerman clan residence. Hearing a knock at the door, Mikasa's father opens it thinking the visitors are Dr.
How The three robbers echoes later: Six years later, as Eren in his Titan form is about to seal the breach in Trost District, he remembers time he killed the leader of the robbers. He thinks to himself that people who reject the freedom of others, no matter how strong they are, do not matter.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.