Yes, you could potentially install each theme into a subdirectory of your site:
e.g.
yourdomain.com <== Your homepage, perhaps with Vantage installed
yourdomain.com/classifieds <== Your ClassiPress installation
yourdomain.com/jobs <== Your JobRoller installation
etc...
You can unify them with a
WordPress multisite installation, meaning your customers only need to register once across all installations, and link the sites via pages.
Thanks.