AT&T just released the Android 4.1/Jelly Bean update for their version of the Samsung Galaxy S III. Rather than the traditional OTA update, they launched this one via Samsung's Kies app. You can download Kies for Windows and Mac here: Kies - Sync Media, Contacts & Calendars Across All Devices | Samsung
Your SGS3 must be running baseband version I747UCLEM, I747UCLG1, I747UCLH9, or I747UCLI5 for the update to take effect. To check that you can go to your “About Phone” in the settings.