If you want just to virtualhost the three domains at apache level, just point the DNS of those domains to your server's IP and tell apache to handle the three domains, using ServerName & ServerAlias directives.
Code:
ServerName primary.com
ServerAlias secondary.net tertiary.org
You can also add aliases in /etc/hosts for your domain name, which will save you DNS requests if you lookup (or your applications do) from the server.
Code:
ip.of.your.server foo.com foo.net foo.org
