Survey3 can connect directly to a compatible flight controller to receive the GPS location data. This means you do not need to connect a GPS receiver to the camera and you can use the drone's GPS data. The drone's GPS information is typically corrected for altitude using a barometer, so it is more accurate than the Standard and Advanced GPS receivers for Survey3.
For the Cube flight controller there are two serial ports commonly used: Telem2 (serial 2) and GPS2 (serial 4).
Here are the parameters to change to allow NMEA pass-through from the Telem2 (serial 2) port:
set SERIAL2_PROTOCOL to 20 (turns on NMEA pass-through)
set SERIAL2_BAUD to 115 (baud rate)
Here are the parameters to change to allow NMEA pass-through from the GPS2 (serial 4) port:
set SERIAL4_PROTOCOL to 20 (turns on NMEA pass-through)
set SERIAL4_BAUD to 115 (baud rate)
You will want to connect the pin3 on the Survey3 GPS USB port to the MCU_TX pin on the flight controller's serial port: