Copy the following code into a bat or cmd file then run it.
reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\XPSP3 /f 2> NUL
reg add HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\XPSP3 /v RCPreview /t REG_SZ /d 1c667073-b87f-4f52-a479-98c85711d869 /f
echo XPSP3 registry key has been set. Please check for updates in Windows Update - Kudos to dailyapps.net
The code above will add a few entries to the Windows Registry that makes Microsoft think that you are a part of the Private beta program. This will mean that when you visit the Windows update site you should see SP3 RC1 as a download.
Note: Microsoft will probably catch onto this.