If you have connected your RX correctly to the PPM input, then you should disable serial RX on all ports since CPPM is not serial.
However if you are using SBUS from your Frsky RX, then yes you would need to enable serial RX on UART 3 (cant remember which one exactly).
Also I do recall with the omnibus F4 V3, that you may need to change a solder jumper to switch between SBUS/PPM, if you send a photo of how it is connected I should be able to help (but best to start a new thread for that)