SQL Server 2005 SP3 has been available on the Microsoft Download Center since 12/15/2008 and is also now available on Microsoft update.
The main SP3 update for Standard, Workgroup, Developer and Enterprise Editions is available as an optional update on the Microsoft Update website and is visible when you choose “Custom” scan. It will be offered for each language and platform we support. All of these updates will be available through Windows Software Update Services (WSUS) also.
Starting in 2 weeks, the SP3 Update version for Express Edition, Express Edition with Advanced Services and Express Toolkit will be available as an automatic update. This update will be automatically downloaded and installed if you have accepted to receive updates through Microsoft Update and have enabled automatic updates on your Windows machine.
More information on SQL Server 2005 SP3 can be found at: http://technet.microsoft.com/en-us/sqlserver/bb426877.aspx
SQL Server Sustained Engineering