The new
epubtest.org site seems to indicate that the only application with some known official epub 3 support is
VitalSource Bookshelf Desktop WIndows, though it is a desktop app.
If you find others, you should add to their test matrix. This site looks like it will be useful!
I agree with the Readium recommendation. That has official IDPF support, so it will be around for a while and is under active development.