How can I setup another phone to backup on my iCloud?
I want to add two other phones to my iCloud +. Can this be done?
All phones using the same Apple ID for iCloud access the same iCloud storage plan.
If you want to share a storage plan with other Apple IDs, see information in Share iCloud+ with your family - Apple Support
If you share a storage plan, the other phones continue to use their own Apple ID and they backup the same way as if they weren't sharing a storage plan but the backup (and everything else they do with iCloud storage) goes to the shared storage plan.