basically i would end up with a total of 5 circuits. there would be 3 15 amp circuits. 2 of them would share a neutral and one would have it's own neutral (the 14 gauge that i am pulling). The new hot 12 gauge would be hooked up to the old 12 gague neutral. of course i would make sure all of the breakers for the multiwire circuits are on opposite 120V legs.
