I'm no expert, but I think you can do this. The only problem I see is that it will take longer to charge all three accessory batteries. I have a two bank charger and in the off season when the batteries are in my garage, I connect one bank to the starting battery and connect the second bank to the accessory batteries that are wired together. For those two batteries I connect the positive to one battery and connect the negative to the other battery.
Another option would be to disconnect the bank that goes to the starting battery and connect that bank to the third accessory battery. Use your alternator to charge the starting battery. This is what I do during boating season and I have yet to have problem with starting the boat in three years. I just make sure that when we are hanging out with the boat off that the switch is turned to the two accessory batteries.
Last edited by moon; 01-26-2012 at 11:20 PM.