AD site architecture

We have a native 2003 AD forest with one domain. We have three distinct
support groups that maintain the several thousands of systems.

We wish to set up three parent sites so that each group can manage their own
systems independently. We might wish to have some child sites for one or
two of the parents. Can we do this all in the same AD domain?

We've heard conflicting advice that there can only be one parent site in an
AD domain and then others who say we can do what I've outlined. Can someone
provide a definitive answer? A link to a Microsoft document would be