The third post, the small one, is the actuation for the solenoid. Generally the main battery terminal goes to the solenoid post and then a smaller wire will go from that to the key/start button and then from there back to the small terminal on the solenoid. The other large terminal then goes to the starter motor active. Most times the wire going back to that activation terminal will go via the interlock safety switches if they are fitted. Depending on how OH&S conscious you are you may choose to leave them out.
