Beta 3 of TFS has now been released and should be available on MSDN today (or very soon :-). The differences between beta 3 and beta 3 refresh are small for most things. Jeff Beehler wrote about some of the beta 3 refresh differences two weeks ago. The most important reason for the beta 3 refresh is that it is built against and uses all of the final release versions of Visual Studio 2005, .NET 2.0, and SQL Server 2005. Beyond that, many of the fixes in it were to enhance the setup experience and to address a few localization issues. So, if you are wondering whether some particular bug is fixed, the answer is most likely to be no, with the exception of some issues that Jeff mentioned.
If you have beta 3 installed, you'll want to read the following guide to succesfully upgrade without losing your existing data.
Migrating to Visual Studio 2005 Team Foundation Server Beta 3 Refresh