I'm not sure you're going to find any reader that has
full ePub3 support for iOS. And perhaps even more to the point: even if you
do find one, it's almost guaranteed that anyone you might want to have read your book won't have it.
If you're just wanting to try/learn ePub3 for your own experience/education.
The Lektz Reader claims to handle ePub3 (iOS, Android, Web).
I'm probably going to try out the
Helicon Advanced EPUB3 Reader (currently
Google Play Store-only), which claims: "Digital books reader supporting the latest EPUB3 standard."