Hello, sorry if this is a common mistake or something that can be fixed very easily, but I am currently building my first quadcopter using a Pixhawk flight controller and Taranis X9D transmitter alongside installing firmware and calibrating through Mission Planner using ArduCopter. It seems as though with my current setup, each motor on the quad is being controlled by one radio channel instead of one radio channel controlling multiple motors. (i.e. throttle modulation seems to only change the speed on the motor plugged into channel 3 on the main PWM outputs on the Pixhawk). If I go into servo output on Mission Planner, each radio channel is assigned to an RCIN channel which I assume might be part of the problem. Please let me know either any resources that might help me fix the problem or if there is any other information I can provide that would be needed to troubleshoot the problem. Thanks!
have you set the servo outputs
SERVO1_FUNCTION,33
SERVO2_FUNCTION,34
SERVO3_FUNCTION,35
SERVO4_FUNCTION,36
and then do a esc calibration
Then have a read here to get you going 
Thank you so much! That seems to have worked. I had changed those values during troubleshooting at some point and had calibrated the ESCs with those outputs, but now everything else is already calibrated and good to go.**