You might be failing to describe the problem you're having
You have just said the PC equivalent of "my car wont' start, what's wrong with it?"
Do you get any error messages? What exactly happens when you try to install SP3? Are you installing over the web or from a local copy of the SP3 installer? Does the System event log show anything relevant?
With these answers, and perhaps more questions to narrow down, folks might be able to assist...
Cheers