Heya
How big of size is the SD card?
And not sure if you are aware of that. Beosound 3 needs directories to work with songs on the SD card. If you simply put music files on the SD card without creating directories it won't work.
EDIT: i did some research, seems like the directory Problem affects only Beosound 2 and 4. But there is something else. If you got Songs with the Cover inside of it, they wont play. This issue only affects Beoplayer 4.65 and higher though (meaning you copy the Songs with the Beoplayer Software to the Beosound 3).