Microsoft has relase a standalone version of Report Builder 3.0, August CTP, you could start it throught the report manager already.
A small stand-alone MSI for Report Builder 3.0, August CTP, is now available for download here: http://go.microsoft.com/fwlink/?LinkID=160384
Download packages for SQL Server 2008 R2 August CTP are available here:
http://www.microsoft.com/downloads/details.aspx?FamilyID=e19689bd-38dd-46c4-8645-f58ca4d61d1f