Hi Gunslinger,
Sorry, forgot all about the three wire question.
The easy way is that the wires can control two circuits, one of them is a common wire, the middle one, and this is +. So you can connect one light to the middle cable and to one of the outside ones, you can also connect another light to the middle cable and to the other outside wire. So when you now push up on a switch, one light comes on, when you push down on the same switch the first light goes out and the second one comes on.
So on one switch you could have say all your running and nav lights on one side and the anchor light on the other side, so when running you put the switch up to turn on the nav lights, when you come to rest at the side of the lake, put the switch to the down position and this turns off the nav lights and turns on the anchor light. Or if you are using the spring loaded both ways, a horn on one side and a siren on the other.
Hope that explains it a lot better.
Your water pump will most probably pull about 10 amps on first kick up so this would definitely require a relay module.
The Robbe numbers you should be looking at are:-
8303 - Relay module
8304 - Reversing relay module
8249 - Solid state control module for controlling a small motor fwd/rev (up to 1 amp)
I have included a scan of the basic circuits, as you can see, they have only used two wires, you can connect another circuit to the spare wire and the common centre connection.
John