The three men had been stranded on a desert island. Desperate for food, they had agreed to amputate their left arms in order to eat them. They swore an oath that each would have his left arm cut off. One of them was a doctor and he cut the arms off his two companions. They were then rescued. But his oath was still binding so he later had to have his arm amputated and sent to his colleagues.
This is often told with a further twist whereby a doctor pays a tramp a large sum in order to amputate the tramp’s arm which the doctor then sends to another man who inspects it etc.