Each theme requires their own instance of WordPress to run, so it's not possible to use all of them on one instance of WordPress out of the box, so the best way to use multiple themes is to install each in its own subdirectory.
e.g.
yourdomain.com <==Your Vantage installation
yourdomain.com/classifieds <== Your ClassiPress installation
yourdomain.com/coupons <== Your Clipper installation
You can share the user database and manage the multiple installations more easily by way of a
WordPress Multisite setup. Thanks.