domain networking

I have 3 music domains

main music domain is multisite wordpress, i want people to be able to visit this main site and when signup be able to setup their own site and choose from 1 of the 3 domains.

this part seems pretty straight forward, i have the WPMU setup on main domain with domain mapping and the multi domain plugins.

the part Im not sure about though is how the other 2 domains fit into the network for the www.

like people will be able to make their site at theirname.musicgroups.ca or theirname.domain2.com or theirname.domain3.com

but what about http://www.domain2.com and http://www.domain3.com how would i map these out.
so they are their own sites. SO if someone visits the main domain they are not just brought to a blank page.

would the easiest way be to setup those 2 sites as users under musicgroups.ca and then use domain mapping to allow http://www.domain2.com and domain3.com

or there is an alternative?

thanks in advance