If you’re looking to run 3 separate domains on the same files but different databases you can do something like this:
http://wordpress.org/support/topic/multiple-domains-using-one-wp-install-not-wpmu
You then have to consider if you want separate uploads folders and style sheets. If so, you’ll have to define a constant so that you can tell the page which style sheet to use and add some code and apply a hook to send each site’s uploads to the correct location. I suppose plugins might also be an issue too, but you could define a bunch of constants in wp-config.php and over-ride defaults if you needed to.