as Quest pointed out, you can convert CS3 brushes (or any other vers for that matter) BUT you need access to an eariler version of PS (ver7...) to open the PSD file.

There isn't a "built in" method to "save as eariler version" like in Illustrator because Photoshop views the native file format (.PSD) the same regardless of the version. the brush engine is quite different between PS vers7 and CS3, so you can't just copy the .abr files (you can from 7 to CS3 but not backwards...)

if you don't have access to a system with PS7 you're gonna' be stuck.
retrocity