Originally Posted by Diesel
No need to isolate the third battery or the forth, fifth, etc.............. The isolation only protects the starting battery from draining while running the stereo. The isolation ensures you will always have a fresh battery for the engine.
Just wire the third battery in parallel to the auxiliary battery. This way the isolator still only sees one big 12 volt battery. I would recommend that you buy two new identical batteries to prevent current disparencies between the hardwired batteries. In addition if I know I am not going to be using my boat for a month or longer I will disconnect one of the terminals connecting the two auxiliary batteries to prevent current flow between the batteries.
Also make sure you do not overload the capacity of your isolation device. Double check the rating and measure your current draw from the 5 amps to ensure the isolator is not overloaded.
Thanks a ton for the info!