Worst customer service experience ever DSL

First fact:  AT&T does not provide nor replace any Battery Back Up (BBU) regardless of what you have or what you were told.

In the 11 years I've had AT&T internet (and TV/phone), I've had a couple of few modems (sorry but I can't remember the model numbers).  The original modem had an EXTERNAL brick BBU.  That modem was replaced by a modem which had an INTERNAL BBU compartment but I had to buy the battery to go in there.  I now have a BGW210 which has NO BBU of any kind.  So I've had the external brick, I've had the internal and now I have no BBU.  Side note:  BBU only runs Uvoice VOIP/phone service now.  If you don't have that, don't even bother.

As is spelled out in section 7d the Terms of Service 

Any backup battery solution is your responsibility. You may choose to purchase battery backup for your AT&T Equipment from third party manufacturers or retailers. For more information and minimum specifications visit att.com/batterybackup.

So you're not going to get any help with BBU.  It's a shame that you were not given the correct information.

You need to call CS and let them know that you will be returning the 2 extra modems you have.  Have the serial numbers on hand of all three so they can make notes on your account which modem you are keeping which ones you are returning.  You have 21 days to get them to a FedEx or UPS store.  You should be given a receipt.  Keep it in a safe place.  This is just another opportunity for there to be a mess up.