I do not remember the details, but you can get a kit for this through the 4x4 supply places, as they use them for their winches.
The two auxiliary might as well be hooked together through a normal "t" connection. They are to just be used as extra backup, yes? If the two auxiliary are intended for the carputer/stereo then they ought to be run to the electronics and only accept incoming voltage from the alternator (charging) but NOT allow the electronics to feed off the main battery. In this way the main battery can be isolated only for starting and the other two will charge but the power will "dead-end" at the electronics so that the main battery is never used for electronics, saved for starting.
I probably made that sound WAY more confusing and complex than it is. Sorry - I have not had a lot of sleep.
You will need one part of one of these kits, a large diode, I think. Anyway, google multiple battery set-ups.
(...or go here - http://www.piranhaoffroad.com.au/pro...or_summary.htm