Has anyone seen a H3 mapping reference around or do we simply have the DTD?

the documentation here http://www.hibernate.org/hib_docs/v3/reference/en/html/ lists all the mapping options under their various categories. It's a little spread out, but it's all there.

Another way is to download the hibernate tools plugin for eclipse. It has an xml editor plug-in that makes sure your syntax and properties for hbm files are all correct