I have exactly the same problem. We have 2 androids (Samsung) and 2 iPhones on our plan. All phones do this update every 3 hours. Both of the androids are set to only update apps when connected to wifi, but it still does it. I called AT&T support and they had me turn off auto-updates from the Google Play app. I did that, plus left wifi off, and it still happens!
This is extremely frustrating, and even fraudelant that AT&T cannot tell me what is going on.
Any ideas would be helpful - short of changing carriers :-/