If you want to add MVC 3 to an asp.net website rather than web project then Scott Hanselman's AddMvc3ToWebForms nuget package will get you 99% of the way there, but will throw an error during install you can saefly ignore (I think, at least in my tests this seems to be the case), and a couple of simple steps are needed after install.
Full details on http://delradiesdev.blogspot.com/2011/08/adding-mvc-3-to-aspnet-web-site.html
Mark (@delradie)
