Correct use of Sub-Domains

Would a correct use of subdomains, but to set up an home page, which would be domain.com and then use admin.domain.com for people to login that work for the website, and then customers.domain.com where customers of there service can login to there info. Is this a correct or realist way to use subdomains, the public (domain.com), the admins/ owners (admin.domain.com) and the customers (customers.domain.com)

Or is there a better way to set up a website for the public, admins and customers? Possibly different domains names for each one? Or just a complete different way?

Thanks for the help in advance.