Galaxy S3 is unable to transfer files over USB: "USB device not recognized"
You may need to use Samsung KIES (software for PC), or get your PC to recognize the 'new' "MTP" USB mode.

further background:

Ice Cream Sandwich supports USB mass storage after all, Galaxy Nexus does not

Wikipedia >> Media Transfer Protocol

Samsung >> SUPPORT >> Kies 2.0



Also see: Can I disable MTP mode and just have a regular USB connection?