So… you want SP3 for XP? Sorry, MS screwed up by not properly testing against one of their enterprise apps and they had to halt the release. What’s that you say? You, as a home user, don’t run “Microsoft’s Dynamics Retail Management System (RMS), a software package for running retail stores”? Oh. Well then, just follow the direct download links on this page and get the copy you want! :) Advisory: its > 300 MB.
BTW, if you feel like virtualizing it, I’ve tested XP SP3 in VirtualBox in Ubuntu Hardy and it works quite nicely. Also, if you have an actual XP SP1 or 2 install disk, you can make a SP3 disk quite easily with nLite (I actually did it in VirtualBox and it worked fine :) I recommend also adding IE7 (the version for “SP2” is the one you want).