Hello, I just connected mine a while ago, figured I would share it for future reviews of this post.
Ports of the RPi and CC3D operate at different voltage levels. The CC3D operates at 5V while RPi GPIO pins operate at 3.3V and cannot tolerate 5V. The voltage difference can overheat and damage the RPi ports. Therefore if you want to communicate the two make sure you have a voltage leveler in between, I used a bi-directional voltage leveler. (
https://www.raspberrypi-spy.co.uk/2018/09/using-a-level-shifter-with-the-raspberry-pi-gpio/) Here's a nice explanation.
Just hook the leveler on a breadboard/prototyping board and connect the CC3D's flexiport cables to the HV (high level) channel while the RPi is connected to the corresponding LV channel.
The flexi port cables are (black,red,green,yellow) for me which is (ground, 5V, tx,rx) respectively.
The RPi has UART pins at GPIO pins 14 and 15 (8 and 10 on the board).
I hooked up green tx cable of the CC3D to the UART tx and yellow rx to the UART rx pin as I said.
In fact from there on I'm not sure how to communicate with the UART pins myself and would love some feedback but this does power the CC3D with the RPi which I guess is the connection you were looking for.
Let me know if you guys want a connection diagram of mine.