Why is ePub 3 support important? From what I have seen elsewhere, there isn't much advantage over ePub2 for an e-ink reader. It adds multimedia support, but that won't be useful on any of these devices.
In any case, the Kobo devices already have some support for ePub3. Kobo have their own format the "Kobo ePub" or more commonly "kepub". This is basically an ePub with a different DRM method and some Kobo specific extensions. These are read using a different reader application than a standard ePub. This application is supposed to support ePub 3. I don't know if all kepubs are ePub3 or only some of them