Bad idea to go ePub 3. Most Readers/apps/programs don't handle ePub 3. So your target audience is going to be so small as to make the effort not worth the bother.
You wont be able to sell to most eBook readers as they won't be able to do a thing with your eBook. Most shops won't accept it. So all you are doing is wasting your time working on ePub 3 if you plan on selling it.