To install the June CTP of the newly rebranded .NET 3.0, you must first uninstall Beta2 first. The directions to do that are here:
http://www.microsoft.com/downloads/details.aspx?FamilyId=AAE7FC63-D405-4E13-909F-E85AA9E66146
There is a link to download an uninstall utility - do NOT use it! It will attempt to remove almost everything from your machine. Just follow the manual instructions instead (which are also at the link above, just scroll down).