Well at least we'll get their proper names or form names if they are forms. Knowing CoroCoro, they'll most likely tease us about the obtaining method or form changing method after revealing their names. Iirc, Shaymin's reveal went like this: Shaymin (Land form revealed, no 'form' name were given by the time) > Skymin revealed and teasing us about the form changing method > method revealed. So our new friends are most likely going to follow the same route.~
I believe it's only three years this time, my reason is mainly because of the number of Mythical Pokémon for this generation. There's one thing I'd love to add though, if they extend the numbers of current Gen VI Pokémon(aka the other 70) then obviously it's going to change the case.
The time gap is a sweet speculation. But if they are really releasing games accordingly with that in mind, some games would end up being too rushed or slowed.