AT&T is rolling out Marshmallow for the LG G3 in the U.S. We've received a handful of tips from owners who have received the update notification, which then bumped their respective handsets up to Android 6.0. Thankfully the wait is finally over and you should receive a similar alert on your AT&T G3 soon enough.
If you haven't yet received the update alert, be sure to give it a number of hours to fully propagate through the system. You can always perform a manual update check by heading into Settings, About phone and check for update.
We can't see any mention of Marshmallow for the LG G3 on AT&T's support website, so this could just be the start of the rollout and we may have a little while to go before everyone on the network receives the sweet release.
Thanks to everyone who tipped us!