How do you install a 3 battery bank on a boat?

DIODES are one way doors for electricity, this is how we will do this.

I assume you want to have 3 battery banks, 2 for the house, 1 for the motors.
The house batteries are already isolated from the engine up front with a diode, first thing to do is find it, splice the cable above the diode, and add another leg of wire and a diode.
IF the diode is inside a heatsink (large hunk of metal with fan blades), you may need to get another battery isolator (name for diodes with fan blade heatsink). The diodes or isolators will allow the power from the alternator to go back to the second bank of batteries, but the house can never affect the engine battery so you can always go home.

If the batteries are of different age, size or type, isolate them. When you set it up this way, you are also very flexible when it comes to buying more batteries. Separate the charging regimen for each bank with one piece of equipment. This 3 bank charger works great. http://www.bdbatteries.com/chargers.php?id=9

When we talk about the ships power system, each battery bank hooks to the input side with a diode to block power from the other battery bank. Call www.bdbatteries.com, or sail into Newport Harbor and have www.coastalbatteries.com help you out.