Gday Chris
If i understand what you are trying to do correctly, there may be another way to do what you want with only one PWM line.
You take the PWM output from one pin, and run it to each of the three Mosfets through suitable load limiting resistors.
Then place pulldown diodes on each line ( from between the mosfet and resistor ), back to individual pins on the PIC.
You can then run background PWM to set general brightness, then by pulling relevant diode pins low, you can effectively turn off those lines. As such yr main loop can control on/off times of each colour line, then adjust general brighness every now and then as reqd.
Andrew