The other users in your family group will be able to add 3 addresses of their own; the 3 you see are just the email address for your root@yourdomain.com account

The other users in your family group will be able to add 3 addresses of their own; the 3 you see are just the email address for your root@yourdomain.com account. User1 will have their own set of 3; login to iCloud.com from one of the family accounts and you'll see.