Microsoft has finally made the Windows XP Service Pack 3 Network Installation Package for IT Professionals and Developers available from Microsoft Downloads.
The x86 version clocks in at 316.4 MB and is intended for system administrators who are required to updated more than 1 computer, or if you don't want to have use Windows Update the next time you clean install Windows XP.
Additionally, Microsoft also released a x86 ISO image of Windows XP SP3 standalone update CD. Currently there is no SP3 update for Windows XP (x64) because the kernel is based on Windows Server 2003 (x64) which has a separate update time-line. For release notes, checked build versions check here for both Vista SP1 and XP SP3.