We know the trio are hiding in these facilities, the only question is, when, or how we'll be able to meet them.

There has always been a new legendary trio set alongside the mascots since Gen ll, there's no way it's different for this generation even if the guide and pokedex "confirmed" that all pokemon have been found (which even they couldn't unlock these doors). Gen 1 Trio was filler, since we all know the game was sort of rushed with most of the attention going to 3D models. Which means, yes, these will be event legendaries, perhaps with the 3rd game (Z/X2Y2) story revolving around them and expanding on Zygarde's purpose/role.

My Observations/What We know:
>>Names for a supposed trio are Volcanion (Water/Fire), Diancie (Rock/Fairy), Hoopa (Psychic/Ghost) that have been leaked recently.
>>The Left most area has what seems to be like heat coming from magma in the ground.
>>The Center area has what seems like Steam or Geysers.
>>The Right area, has giant fans/windmills.
>>Access to each of the doors in front of these areas are locked, with no way to get in that has been discovered as of yet.
>>When getting near these areas, the camera PANS toward them, they wouldn't just add that for no reason, there's significance to these places.
>>The strangest thing is, the rumored poke's types don't match up to these facilities with the exception of Volcanion, but a certain trio does. Look back at those facilities, now think of a trio. This could be a hint at Groudon, Kyogre, and Rayquaza rather than a new trio.

So let's speculate/discuss while we shiny fish/Breed pokes!!