Here is how you can download SP3 straight from Microsoft.
1. Download XP SP3 Hack by dinguskull.
2. Once you have downloaded the file, login as administrator and run the file by double clicking it. Wait for the Confirmation Message on screen.
3. Check for new updates on Windows Update. You should now see Windows XP SP3 listed in the available updates.
5. Download and install SP3. Reboot whenever necessary.
Note: Service Pack 3 for Windows XP is still in beta. Use at your own risk.