I currently have "AT&T LTE 2" as the selection under Settings/Access Points.

For the Cellular provider it was a bit weird in that the first time I went in there I saw the same thing you did, 3 values of "AT&T". But after doing that change in Access points when I went back I only saw one "AT&T" value and selected that.


 Edit: I did find some other info on a thread here that I found interesting, so passing it on: