Using diodes to wire a battery pack in series and parallel for different voltages
The diodes stop the batteries from shorting to each other, but they also deliver 36 V to your '12 V' output.

If your low voltage drain is very, very small, say less than 1% of the drain on the whole pack, then you could maybe supply it from one battery, and rely on the charger to rebalance the cells when you recharge. 

Best to use a DC to DC converter running from the whole pack to generate your low voltage supplies.