Hello dear..
Although, i am not a professional tech person. But, still want to assist you some efficient ways to install XP SP3.
For this, firstly you have to turn off automatic Updates and for this write services.msc command in "Run" dialog box. Then clear the Software Distribution Folder. After that turn on Automatic Updates to download Windows XP SP3 again. And finally, reinstall Windows XP SP3.
Moreover, if you want to get elaborated description to install XPSP3, you can go through
http://www.completepcpedia.com/service_pack.html
Hopefully, it will provide you solution to install SP3 in your Windows XP operating system.
Thanks