How do you connect 3-12 volts battiers on a 1985 southwind motorhome?

Well, that depends.. are you looking to make a 12v circuit or a 36 volt circuit? Presumably, these batteries will be for powering the accessories in the camper compartment, rather than the cab compartment. Honestly, without a wiring diagram available and without knowing what kind of circuit you're trying to wire up, you're better off to go to an RV service shop.