two options, I prefer the second link myself. It's a awesome little utility that is very well laid out especially created for SHIII, it unpacks all the Dat files contained within SHIII and creates it's own folder to store them in.
http://www.filefront.com/14846731/Dat-Extractor.rar/
http://www.filefront.com/14985481/SHIII-Unpacker.rar/
I hope it's what your looking for.