The EPUB3 format is the "next generation" for the widely-used EPUB format. (the differences between this and EPUB2 are described in this question).
Which devices today support EPUB3? Where can I find out information about EPUB3 support?
http://epubtest.org/ is very current and provides stats on overall eReader support as well as support for individual ePub 3 features. It would appear this site is updated regularly and is likely to continue to be current.
There is no one reader that works across platforms. For a large and fully-loaded ePub3 ebook, the following readers will work pretty well. No reader is perfect, but these ones have minimal issues.
MacOS X: iBooks, Adobe Digital Editions, Readium (in Chrome browser, but only with unzipped epub3 files)
iOS: iBooks, Adobe Digital Editions
Windows: Adobe Digital Editions, Readium (in Chrome browser, but only with unzipped epub3 files), Azardi Reader
Android: Adobe Digital Editions
ChromeOS (on a Chromebook): Readium
I did try quite a few readers for this test. Need to do Linux, but maybe next week! Tested ePub3 book is about 900 pages, with video, slideshows, audio, images, etc.
BTW, treat the 'tests' of readers with a grain of salt. Gitden did very well on the tests, but cannot handle fixed layout files with embedded fonts, even the version updated to fix this issue. It also has a number of other issues, especially with large books.
The Infinity Reader on Android supports fully EPUB3 format. The app is distributed with a book, which has embedded youtube video.
You can use Readium extension on Google Chrome browser to read EPUB3 book on Windows or Mac. To use Readium app, open Chorme Web Store and search for Readium.