The latest edition of PCA (received today) and the home page of this website states how to get SP3 but as far as I am aware SP3 final version isn't available as of now and it isn't downloadable as described by PCA yet (I've just checked). I would do as johnnyrocker has stated. SP3 final version will probably be worthwhile having.
You have another option in that you could use the registry script (only 38K) and available here: click here Unzipping it installs it into your registry and then by going to the normal MS Update site it will select/download and install only those bits of SP3 which are relevant to your circumstances. The full 336MB beta version contains absolutely everything since SP2 - you will already have a lot of it through the normal update process - and some of it applies to processes which you may not even have installed. My download by this method was only 65MB and I am hoping that (as the link above also asks for the removal of previous SP3 releases before applying it)that come the final release it will just require updating and not uninstalling/reinstalling