Step 1. Hold down Volume Down, Home Center, and Power buttons together for about 20 seconds until your phone resets and shows you an option to get into download mode.
Step 2. Next hit Volume Up button to enter download mode.
Step 3. Once in Download mode, you can connect your micro-USB cable from your Galaxy S3 to your computer.
Step 4. Next, download GalaxyS3RootSprint.zip and unzip, you should find all the files you need.
Step 5. Next, make sure you have drivers installed, you should see “Samsung Mobile USB CDC Composite Device” in Device Manager. If you don’t, simply download Samsung Kies and install it. (Then unplug, plug the micro-USB cable.)