please be aware, that all portals of an installation are independent in administration, user base and content. Child portals are same as host portals, but reside inside a subfolder, e.g. www.myportal1.com/myportal2. AFAIK you can nest 3rd, placing it in a subfolder like www.myportal1.com/myportal2/myportal3, by I do not see an advantage.

U can use a single portal and implement the organizational structure in the page structure and role names, but try to get the real requirements first:

• which data shall be provided by the portal
• who shall enter the data / how are responisbilities for content defined / distriibuted
• which are target groups (internet/intranet/extranet)