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."