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 some equipment using unbalanced signal and accepts a XLR connection, pin 1 and pin 3 shall be connected.

Bibliography: AES14-1992 Standard