Third level domains are registered on your provider's DNS server. You don't need to register a third level domain with a domain registration company. If you choose to create a third level domain, it will contain your provider's domain name. E.g. if your provider's domain name is besthosting.com, and the third level domain you are registering is mythirdlevel, the fully qualified domain name would be mythirdlevel.besthosting.com.
Third level domain registration is available only if it's allowed under the selected plan. Functionally, third level domains have same features as second level domains.
To register a third level domain name, enter the desired domain name in the Domain name field. Choose provider's domain name to the right and click Submit.