I have 12/3 wire from a switch to 3 light fixtures; can I feed the switch with 14/2 wire from a 15 amp breaker?
Yes. There is no problem using 12 AWG wire on a circuit protected by a 15 ampere circuit breaker. If the lights; or anything else on the circuit, tries to draw more than the #14 wire can handle, the breaker will trip.
There's no problem, and it is permitted by code. However to prevent future confusion I recommend marking the 12 gauge wire with a permanent marker or tape: "15 amp circuit". That way nobody in the future gets confused or assumes it is a 20.