In the domain name system (DNS), third level domains are the next-highest level of the hierarchy underneath the second level domains. In a domain name, it is that portion of the domain name that appears two segments to the left of the top-level domain. For example, the "sterling" in "sterling.va.us." In the case of the domain name "www.yourdomain.net", the third level domain is "www."
Third-level domains would allow you to use our domain name in the format http: //yourbusiness.zerolevity.com. Often this might be preferrable to smaller businesses that don't wish to pay InterNIC fees for a domain name, yet don't want a long website address like http: //users.zerolevity.com/~username.