From what I can see, the file structure of HTMLEditor3 differs to some extent from HTMLEditor2 and the way that user styles are created and written to file is different too.
If you began applying HTMLEditor3 to your CMS, it would create a new folder (webassist/ckeditor, webassist/kfm) so you would have a whole new set of files. It is likely (though Ray or Jason may correct me) that you would have to reapply your existing bindings to each new HTMLEditor3 instance, and perhaps juggle some paths.
If you have created a CMS using HTMLEditor2, it would be best to continue with that to completion - so you have a finished working version online, and then perhaps you could duplicate your local site and play around a bit with the new version on your local testing server if you then wanted to.
