1)Setup your 3rd channel program mix
Make the 3rd channel independent from 1st and 2nd channel. You can then set the default starting/ending point of the 3rd channel.
2) Assign the program mix toggle to one of the buttons or switch on your 3pks
Set it to toggle mode, so you dont need to hold the button to activate the 3rd channel
When you press the button, it will trigger the program mix, which your 3rd channel servo will move directly to the ending point you set in (1)