By chance are you all using iMessenger? if so , this could be where the data usage is coming from. imessenger will use data instead of text messenging when send messages from idevice to iDevice. As for the updating every 3 hours. At&t doesn't do live, up to the minute , data usage updates and this is why you see the data change every 3 hours. I hope that this little bit of information helps.
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