As you have probably seen, SQL Server 2005 Service Pack 3 is available for download at http://www.microsoft.com/DOWNLOADS/details.aspx?familyid=AE7387C3-348C-4FAA-8AE5-949FDFBE59C4&displaylang=en.
At the end of January SQL Server 2005 SP3 will be available through Microsoft Update. SP3 that will update non-Express editions (Developer, Enterprise, Standard, and so on) won't be available for automatic update. You can obtain the update by opting into Microsoft Update (http://update.microsoft.com/microsoftupdate/v6/default.aspx?ln=en-us) and then scanning. SP3 that will update Express editions (Express, Express with Advanced Services, and Express with Toolkit) will be available through Automatic Update.
Once the Microsoft Update logic is published you can use tools like SMS and SCCM for deployment.