XLR connector is a balanced connector for audio signal with 3 pins: Positive Signal, Return Signal and ground.
Pin designations: Positive Signal: pin 2, Return Signal: pin 3 and Shield: pin 1.
These connectors have 3 pins and are used for balanced connection, but if a piece of equipment uses an unbalanced signal and accepts an XLR connection, pins 1 and 3 must be linked.
Bibliography: AES14-1992 Standard
