Step 3. When you see a warning screen like below, hit Volume Up button.
Step 4. Once in Download mode, connect microUSB cable from your AT&T Galaxy S3 SGH-I747 to your computer.
Step 5. Download GalaxyS3RootATT.zip and unzip, you should fine 4 files and everything you need to root your AT&T Galaxy S3 SGH-I747.
Step 6. Double-click on the file “odin3 v1.85.exe” to start the ODIN program. You should see a yellow-highlighted box with a number. If you don’t, download and install Samsung Kies, then unplug/plug your phone back in; you should see it now.