How do I specify a parent domain in a multi-site install?
It is called multi-site aliasing, hence the results. Drupal offers you to server multiple domains from the same folder in order to preserve the paths in the database. So you would use the same folder for example.com and sub3.com because you might want to disable one of them later on. Aliasing is meant on a file level here. What you need is a redirect.

A 301 redirect code can be issued by your webserver-software itself (e.g. Apache/nginx config or apache .htaccess) or by sending a header via the scripting software.

The "drupal-way" solution for you would of course be to use a module for sending that header. My recommendation would be multisite_redirect, because it would only need a one time setup for your case outlined above.