The official release page for Windows 2000 Service Pack 3 is at http://www.microsoft.com/windows2000/downloads/servicepacks/sp3/default.asp.
Microsoft Knowledge Base Article 320853 contains the following summary:
This article provides a list of the bugs that were fixed in Windows 2000 Service Pack 3 (SP3).
Service packs are cumulative.
This means that the bugs that are fixed in a
service pack are also fixed in later
For example, Windows 2000 Service Pack 3 contains all of the fixes that are included in Windows 2000 Service Pack 1 (SP1) and Service Pack 2 (SP2).
You do not need to install an earlier
service pack before you install Windows 2000 Service Pack 3.
Click to view a list of the fixes in Windows 2000 SP3
If you use Internet Explorer 5.01, Internet Explorer 5.01 Service Pack 3 (SP3) for Windows 2000 is installed when you apply Windows 2000 SP3. For additional information about the fixes that are installed, click the article number below to view the article in the Microsoft Knowledge Base:
320665 List of Issues Fixed in Internet Explorer 5.01 Service Pack 3 for Windows 2000