So far a 3rd floor is not possible. I know people in the official forums asked if that would be something that could be added to the new version, but so far no officials responded to that. I think even people who play the Chinese/Japanse versions (V4.0 and 5.0 there) said that a 3rd floor is not possible yet......

I do hope though, that they'll include it someday. With all the new furniture they've implemented, it would be good to have a third floor too.