The most official remaining source is the Microsoft Update Catalog (KB936929) , which provides the 319.3 MB executable (EXE) file.
While original XP requirements were lower, SP3 "weighs" more on the system due to new background services. How do I download and install Windows XP SP3? download windows xp service pack 3 english
Because Microsoft ended official extended support for Windows XP in 2014, direct downloads from the primary Microsoft homepages are often redirected to newer OS versions. However, reliable repositories still host the official standalone installer: The most official remaining source is the Microsoft
Sites such as Softonic host mirrors of the installer, though users should always verify file integrity. System Requirements for SP3 download windows xp service pack 3 english