Third level domains are created based on second level domains and have same features as second level domains. Customers can use your service domain to create third level domains of their own. For instance, if your second level domain is example.com, your customers can create domains like illustrative.example.com.
Third level domains are registered on your DNS server and need no domain registration company to be registered.
To add a domain for third level hosting, you need to create a DNS zone and allow third level hosting in the zone settings.
Third level domain registration will be available for your users only if you allow it in the plans. See Plan Creation Wizard for details.