Did you connect the neutral from the 14-3 to the AFCI or is it still connected to the neutral buss in the panel? If it is connected to the breaker you probably have a ground fault on the neutral somewhere in the circuit or another circuit is sharing one of your circuit conductors. Disconnect all 3 of your circuit wires in the panel and check continuity between each wire and ground. DO THIS WITH ALL POWER TURNED OFF NOT JUST YOUR CIRCUIT! If any wire shows continuity to ground you have a problem and need to start searching.
Curt
