Ideally you would want to feed the switches hot and then from the switch run a 12/3 with gnd.
to the fan/light combo then out of the combo with a 12/2 with gnd. To feed light one and three.
Light circuits are almost always 14 guage wire, which is good for a 15 amp breaker! 12 guage wire is usually reserved for wall plugs, and using 12 guage wire for light circuits will always confuse the inspector, and is overkill for almost any common household lighting!!