I can't think of a valid reason to uninstall SP3, but it does have an uninstaller.
Go to Add/Remove Programs in the control panel, put a checkmark next to Show Updates at the top of the dialog box, and then scroll down to "Windows XP Service Pack 3" and click its Remove button.
If it isn't there, then the uninstaller has been removed, probably by a program that claims to regain space by removing unneeded files.
Good luck,
Llama