I think you may have left the issue too long now to be rectified (4 Months later) how did originally set up the Plans and Numbers, Online, EE Store, by Phone etc, the only recourse you may have is if you did this by Phone to EE Customer Services and are confident that EE somehow got it wrong, as the Call would have been Recorded and can the conversation can be checked by EE.
However, all is not lost, if the 3 Numbers are all on the same Account, you can simply transfer the Monthly Data Usage between the Numbers by Data Gifting, you would need to do this every Month but its a very easy process and will take less than a minute to sort out.
