Yes, it does.
If you want to do a quick test, download one EPUB 3 from our (demo, free) projects, for example:
1) eBook:
http://www.smuuks.it/index.php/en/pr...ivina-commedia
2) Audio-eBook:
http://www.smuuks.it/index.php/en/pr...f-lino-pertile
These are EPUB 3 files: they are displayed correctly in iBooks. In the case of the Audio-eBook, as noted above, since they are declared as "reflowable", Media Overlays feature does not work in iBooks, while being correctly handled in Readium.