When I heard the news I didn't expect it to matter much; I thought it would be months before we saw any apps or ereaders that supported the new format. Apparently I was wrong.
The app isn't available to the public, unfortunately, because Access are looking to license it to other developers so they can use it in an app or on an ebook reader. Like the NetFront Book Viewer, which Access release 2 years ago, NetFront BookReader is going to be available with another company's brand on it. You'd probably only know that this app is in there if you checked copyright page.
Now, I got the pres release last week and I held it because I wanted to name some of the developers who are using it. Access didn't share that info, so I'm going to ask my readers if they know whether anyone is using this app.
If you're interested in looking at an Epub3 compatible app, check out iBooks and try one of the demo ebooks I found a few months back. iBooks is mostly compliant with the Epub3 standard, but Apple never released full details on what they do and do not support. Apple also added their own quirks to iBooks which might have some interesting effects.