I agree with Ian that a brushless DC motor would require more than 3 wires if properly installed. It is possible that just 3 wires are used, one per phase, but that would indicate the motor is not grounded. That is a safety (and probably EMC) hazard.
A link to the motor specification or supplier would be useful. Is it a sewing machine type?
If this is a connection between a motor drive and a brushless DC motor you should not use a standard C13/14 connector or similar as there is a possibility that someone could plug it directly into the mains. You would also have power on the “earth” conductor, a big no-no. There are connectors intended for this aplication but they are not low cost.
Robert.