Microsoft Service Packs include updates from prior service packs. That is; SP2 includes SP1, SP3 includes SP2. I use Autostreamer to slipstream my Service Packs. The procedure for Autostreamer is described here: http://www.simplyguides.net/guides/using_autostreamer/using_autostreamer.shtml
We assume you've downloaded SP3 from Microsoft: http://www.microsoft.com/downloads/details.aspx?FamilyID=5B33B5A8-5E76-401F-BE08-1E1555D4F3D4&displa...
As with Nlite, the Autostreamer slipstreaming process is fairly straight-forward.