Here’s a little video I’ve just done on installing Windows SharePoint Services SP3 on Small Business Server (SBS) 2008.
http://www.youtube.com/watch?v=IIEYRRHyUME
Pretty straight forward really but remember that you need the 64 bit version of the Service Pack which is at:
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=27828
and it will automatically run the configuration wizard. Manually running the PSCONFIG command is only for updates on SBS 2011 and SharePoint Foundation 2010.