When you try to preview a report inside Report Builder 3.0 you may see an error message similar to "Failed to preview report. The item [path to server] cannot be found. See the attached image for the full error message. This behavior occurs when Report Builder 3.0 is in connected mode but the current report has not been saved for the first time and Report Builder is using temporary paths and file names.
You can see in the status bar the server which Report Builder 3.0 is currently connected to.
Save the report to a SharePoint document library then run the report.
SQL Server 2008 R2 Report Builder 3.0 and a Report Server in SharePoint integrated mode.