rule bilge pump wiring
The third wire indeed goes to the manual/automatic switch. If you don''t have a switch, you can terminate this wire with a waterproof connector (keep it out of the water just in case though).
As a side note, you should run the power wire directly to your battery or the hot side of your battery switch. This is very important, as you want your pump to be operable when the battery is switched off. Don''t forget to include an inline fuse of the appropriate amperage as close to the battery as possible. The Rule 500 draws about 2 amps and should therefore have a 2.5 amp fuse.
If you really want to get fancy, you should wire in a bilge pump cycle counter so that you know how many times your pump has turned on when your gone. This will tell you if the leaks in your boat (all boats leak) are getting worse. A seperate high water alarm is also nice to have.