I have two devices that fully use icloud and I only have around 2 GB used on icloud. She has an average of 5 GB per device, that's a lot. If she has lots of photos, then she should be syncing them to a computer for archiving and not bothering to back them up in icloud. (Always sync photos to a comptuer for archiving.)
If she has a lot of text messages, the those are automatically backed up to an icloud backup - have her delete many of them. Videos? These take up a lot of space. And, yes, if she has duplicate data on the devices, they will be backed up multiple times - nothing can be done about that, unless she has only one device that backs up some of the redundant data and not the others.
As I said, using 15 GB with 3 devices is a whole lot of data.
This link gives an overview of backup and restore:
http://support.apple.com/kb/HT4859?viewlocale=en_US&locale=en_US
Go to Settings>iCloud>Storage & Backup>Manage Storage. Choose the device you want info on. On the next page, you'll get a listing and size of all data store in the next backup. You can also turn on/off backups of individual apps' data.
And photo stream doesn't use up icloud storage in your account, so don't bother dealing with that. BTW, the stream is used to sync photos betwen devices, don't use it as a storage service. Always sync photos to a computer for archiving.