Here's what worked for me after literally trying everything under the sun (compiled steps from various sources):
Steps to Safely Cleanup and Remove old USB Mass Storage Drivers on your PC
Shut down your computer. Unplug your USB storage devices: USB Disks, flash, cams, CD/DVD, etc.
==> Other USB devices like keyboards, mice and LAN adapters can stay plugged in
Boot computer back up. Click for how to determine whether you are running 32 or 64 bit Windows
Download drivecleanup.zip, unzip and save to your desktop. Move the 32 or 64 bit version of DriveCleanup.exe toC:\Windows\System32
Open a command prompt window as follows:
==> For XP only: You must have Admin privileges. Click Start->Run, enter cmd
==> For Vista and Windows 7: You must open an elevated command prompt window!
After the command prompt enter: drivecleanup.exe
==> Your command prompt window should be similar to my example below.
==> If, instead, you get an error message like Command not recognized it means you didn't move Drivecleanup.exe to the right directory
Reboot.
Download the latest USB drivers here:
Support - AT&T Cell Phones SGH-I747 | Samsung Cell Phones Click the tab at the bottom that says "Manuals & Downloads" then Software then the link for the Device(Install),USB Driver (Software) (ver.v1.5.14.0)
Intstall Drivers
Reboot
Unplugged or disable your internet connection ***CRITICAL***
On your S3 go to Settings>About device>Check for updates and install the update (eventhough we next do a hard reset the new firmware is worth it if only for the brightness feature added to the notification drop down).
Do a hard reset on your Galaxy S3 instructions for that here:
Samsung Galaxy S3 hard reset - PDA Smartphone PNA hard reset soft reset
Turn on S3 (without USB plugged in) and don't login to your Android account ***CRITICAL***
Plug in USB and it should install all the drivers at long last ***SIGH***
Login to your Android account with the USB plugged in and you should be good to go!
I hope this works for you because I went through probably 8hrs of B S trying to figure this out. Believe when I say I tried EVERYTHING and this is only fixed it! Oh one other nice thing is this fixes the annoying COPY/PASTE issue as well. I know it's pain in the neck but it's worth it.
I'm on a Window 7 Ultimate Service Pack 1 32-bit machine.