The ESC (Electronic Speed Controller) allows the speed control of the motors according to the orders of the flight controller. Typically the control signal used by the ESCs is a PWM (Pulse Width Modulation) but some models allow the use of I2C or CAN signals.
The Pixhawk 3 Pro has 8 PWM outputs for 8 ESC / Motor pairs.
Please connect each of your PWM ESCs to the corresponding "Main Out" output. Refer to the diagram below for the order of your ESC / Motor.
All blue motors should only be used with CCW propellers. Green motors with CW propellers.
To see what a CCW propeller looks like, please see the previous picture.
Also check the direction of the motors.
For safety reasons, remove the propellers before handling!
To reverse the direction of a motor, simply swap two of the three cables that link the motor to the ESC.