Figure 23-3 in the data sheet shows the key timing variables and the duty cycle itself is discussed in Section 23.3.5. Given Equation 23-3 and that you generally need to keep the PRx register value fixed, you should be able to derive the formula to convert the duty cycle into the required "CCPRxL:CCPxCON<5:4>" value and set the registers accordingly.
However, before starting on that, you need to make sure that you are reading the potentiometer value correctly through the ADC. I suggest that you start with a program that initialises the ADC and, in the main loop, simply reads the ADC value for the pin connected to the potentiometer arm. One you can do them you can then convert that value into a percentage (or fraction) of the full scale value. Of course this is really the duty cycle.
The next step would be to then use that value to calculate the PWM value as mentioned above for one of the PWM devices. Once all that is working it should be very easy to extend the duty cycle setting to all 3 PWM peripherals.
Susan