See also
Okay, this one is quite clearly stated in the documentation but I think I failed 3rd grade reading, so writing it down anyways.
https://ardupilot.org/copter/docs/common-servo.html#connecting-the-servo-to-an-autopilot , specifically “For a Tricopter configuration” paragraph
When building a tri-motor tilt-rotor, note that motor 3 is skipped in the channel output. Assign outputs for motor 1, 2, and 4 — not 1, 2, and 3. If not using tilt-for-yaw, then motor 7 is for the tail tilt servo.
The symptom of getting this wrong (assigning motors 1, 2, 3) is a prearm failure for “No SERVOn_FUNCTION assigned to Motor4.”
See also
For a tricopter, also note that due to PWM rate servo groupings, no servos should be assigned to channel 4. See Servo control rate groups.
Motor ranges also need to be calibrated. See Tilt servo ranges.