Did you manage to write any file on the SD card? Has your file manager the permissions to read these files?
Perhaps your SD card is not compatible with the FP3. Please check if it as been reported to work in the following thread: ✏ List of compatible microSD / SDXC cards for FP2 Edit: Whoops, that thread is for FP2.
Your cable should work. But please test another one, you never know.