Yes, and that is a very good way to approach that. I was responding to your original post regarding *components" for Joomla! - if you want to run WordPress and phpBB, you can certainly do that from within, or from subdirectories under, your Joomla! directory.
I think doing it in subdomain can be simpler for a new user because you can still use the "one-click" installer and don't have to worry about possibly conflicting .htaccess files.
To do what you suggest, just Create the new subdomains from the Control Panel -> Manage Domains screen and wait for them to become "live", then use the "one-click" installer just as you did with Joomla!