Re: How do I conect 3 batteries?
ok chose which battery you wish to add the backup battery to, then all you have to do is wire positive to positive and negative to negative, thats called parallel wiring between the batteries on the side you want to add the backup to. Use a fairly sizable wire I prefer 4 gauge and seal the connectors properly with heat shrink that has heat activated glue. when I do this I use batteries of the same age in a bank(two or more batteries wired in parallel is a bank) but others do not its your choice. Your done, now either switch batteries while running to recharge the batteries or run in the both position again your choice.
Probably more info then you wanted but if you were to use a jumper between one batteries positive to the other batteries negative and then your load lines to the other ends of the batteries would be series wiring and be 24 volts but you have no use for 24 volts in your application
Also your charging system is fine for this application but without the added information we have no way of knowing that we are giving you the proper advice for your boat, as Silvertip alluded to