It could be a pulse encoder - one pulse that the PLC counts to determine the speed. This would be my 3-wire guess.
It could be a speed generator, providing a 0-5 or 0-10VDC signal the PLC uses to determine the speed. These usually only have 2 wires though.
It could be a speed generator, providing a 0-5 or 0-10VDC signal the PLC uses to determine the speed. These usually only have 2 wires though.