I spotted ASP.NET MVC 3 for Visual Studio 11 Developer Preview on Microsoft Downloads last night. I haven't installed it yet so I don't know if you can install it side-by-side with ASP.NET MVC 4. My guess is yes, since we've always been able to install different versions of ASP.NET MVC at the same time in Visual Studio.
Download it now.
I have added this download to the list of ASP.NET MVC 3 Tutorials, Links, and Downloads.
David Hayden