One wire is for the light, one is for the operation of the spout and one is common. The original colors for the wires were yellow, green and black, respectively. You may need to do some trial and error if it has been rewired.
Hook the common wire (black) to your common post (usually U)
Hook the light wire (yellow) and another convenient length of wire to your voltage post (A,B,C,D). The other convenient length is attached to one terminal of the button.
The operation wire (green) goes to the other terminal of the button.
The light should be constantly illuminated, and the button should control the spout. 12-14 V is usually adequate. I answered a similar question here.