What's this? A current-generation AT&T phone and its year-old sibling getting updated on the same day? We have an embarrassment of riches, folks. Just a few hours after the Lollipop over-the-air update for the Galaxy Note 4 started on Ma Bell, the Galaxy Note 3 (from 2013) is getting the same treatment. AT&T's support page says that this is Android 5.0, build number N900AUCUEOC1.
As AT&T does with all their updates, the OTA will only start on Wi-Fi (and not that mobile connection that you're paying them to provide). The OTA file is a massive 1.2 gigabytes, and whatever server AT&T uses for these things probably has some tiny Flintstones dinosaur manually pushing data packets down a CAT-5 cable. It's slow, is what I'm saying here. If you happen to get the update alert in one of the early batches, you might want to let it come down overnight.
The update message doesn't mention anything beyond the standard Lollipop features, though Galaxy Note 4 users mentioned some additional downloads for Samsung's voice recognition software. As always, don't be surprised if it takes a few days for the update to appear in your notification bar - if you simply can't wait, check XDA, SamMobile, or your user forum of choice for a manual download.