Windows XP Service Pack 3 Release Candidate 2 is available to the public. Specific registry settings will allow you to be offered SP3 via Windows Update. You can also choose to download standalone update packages.

Read and download it from Microsoft download center

Note : To obtain RC2 via Windows Update, download the script and run it on a machine currently running Windows XP Service Pack 2. The script sets a registry key on your system. The registry key is required for Windows Update to recognize your machine as a valid target for Windows XP Service Pack 3 RC2.

check more at Microsoft Download center