Right now, the over-the-air update for the AT&T variant of the Samsung Galaxy S3 is available for download and install. If you haven't yet received a notification to update, tap through Setting -> More -> About device -> Software update -> Check for updates to initiate a manual check. This comes a little over a month after the Sprint variant received its update.
Keep in mind that OTAs are staged out, so if you haven't received yours, it's coming. And of course, if you have an unlocked bootloader or a custom recovery installed, you'll either need to remove those, or manually apply the update when it becomes available (in the case of the Sprint update, this was a few days after the OTA).
If you aren't rooted now but have an inch to see what all the fuss is about, take your OTA update, then root using the easiest method we've ever encountered. Seriously, one click and thirty seconds later, you're done.
We'll have a guide up for rooted and unlocked/custom recovery users shortly, so stay tuned. After that, you can apply the new Galaxy S5 toggles to your device, and give new life to our aging flagships.