Porsche 914 EV Conversion: Battery Equalizer Schematics Rev 1.0
could use that circuit. Just hook the charger to a middle battery and let the circuit balance them out over time. If nothing else just double the capacitors up or go to much larger ones so you get more current flow between the batteries.
That is one of the things I am considering doing for my car so I can use solar to charge the 12V battery and the 72V pack at the same time.