Listen ya Gewber. Imagine them red and black wires runnin to them two empty battery post. And it don't matter ifn the three batteries are sittin in yo boat or in yo livin room. Ifn they in yo livin room, you mite need 4 gauge, otherwise ifn they somewhere in yo boat, 6 gauge will werk. Ifn you done got the 4 gauge ran, by all means Gewber, don't pull it out.
As far as yo three wires, borrow a volt meter and put one lead on black and then check each red wire. Should be 36v on one of the red/black combinations.
You gonna fewl around and smoke it ya Gewber.