What is a safe download link for Windows XP Service Pack 3?

Service Pack 2 and Service Pack 3 are available from Microsoft here: http://catalog.update.microsoft.com/v7/

You must use Internet Explorer 6 or later.

Search for "XP Service Pack" (without quotes)

Note that SP3 itself is quite old by now; even with it installed, you'll still need to check Windows Update (if it still works) for a few more years' worth of patches.

Archived SP3 download links from the official website are:

• Digitally signed EXE installer (Authenticode):
  WindowsXP-KB936929-SP3-x86-ENU.exe
  SHA1: c81472f7eeea2eca421e116cd4c03e2300ebfde4

• ISO image:
  xpsp3_5512.080413-2113_usa_x86fre_spcd.iso
  aka en_windows_xp_service_pack_3_x86_cd_x14-60489.iso
  SHA1: 8fa76ccea145d050fc6a506ffbdedfe53282e5b4