[fjtorres]

Nothing supports epub3 fully or even mostly.
Many promises, no delivery. Not even the adobe reference implementation.
Indications are it is at least a year away.
Unless your're into math-heavy technical papers you won't see much benefit from it when it finally appears, anyway. The really useful aspects have already been grafted to the propietary formats like iBook, kepub, and kf8.
Get whatever reader appeals to you but don't hold back over epub3. It's not a meaningful discriminator.