I'm hoooking up a bathroom heater/vent/light in an addition and ran a 12/3 wire for the heater and fan on a dual switch. A separate 15 amp wire for the light on a single switch. Both circuits and switches in the samebox without a divider.

Are you saying you ran a 12/3 cable from the circuit panel to the switch box for the heater and fan and ran a 14/2 (on a separate breaker) from the panel to the switch box for the light?