I know Adobe InDesign is a commercial epub3 editor. Is there any open source epub3 editor?
Actually, Indesign does not yet export ePub3 files and it never worked as an editor for existing ePub2 files. I strongly suspect InDesign CS6 will export to ePub3 when it comes out late Spring. For now, Sigil is great for editing ePub2. The closest thing to ePub3 publishing right now is probably the (expensive) toolset from InfoGrid Pacific. You'll have to wait for the ePub3 spec to get implemented by eReader manufacturers and software makers but Infogrid's free Azardi reader will give you a good preview of what's possible.