Pixhawk, fs-th9x and 4 servos?

hi i have an Iris with retracts and im trying to add pan,tilt,roll servos.
so far the retracts work on txs gear switch, ive got pot 1, and pot 2 to work but i cant get pot 3 to work.
do i need to enable ch.8? im soo stuck, has anybody figured this out with the stock transmitter or do i need to get a Taranis?
thank you

Yes you should be able to assign another mix for the auxiliary channels.