SD Card would be the actual external storage, i.e, the memory card. The other two are just partitions in the built-in memory. 2gb for Device memory and the rest as USB. Not a very great idea from Samsung I suppose

Device memory is reserved for system apps, while the bigger partition will be for your media files. Try using a file manager and see if you can find the sdcard folder, it will probably be the same as the USB storage you are referring to.

Try installing free apps like App2sd or similar to see if you can move apps to the external storage. Not all apps can be moved though.