Trouble with Motor 3

We have been having trouble with Motor 3 for a while. Before when we tried the motor test, moving the motor 2 slider would cause both motor 3 and 2 to spin. This caused the drone to flip during our flight tests. We believe we have fixed the issue with motors 2 and 3 being in sync, but now we cannot spin motor 3. We have received a second FC and a second ESC, any solutions would help.

check to make sure that the red wires on the harness to the FC are not reversed, we had that problem and motor 3 was intermittent.