Kingdom Hearts 3 - It's been 10 years,

"Kingdom Hearts 3 have been an upcoming game
with an unknown release and platform,
There have been a lot of speculation and rumors on the game platforms and when it would be released for the past 6 years but the game is this not on the shelves.Another reason for this may be due to the series creator Tetsuya Nomura's focus on Final Fantasy Versus XIII.

There was a lot of rumor over what platform the game would be on as The Official Playstation Magazine mentioned a possible released date (2009) and said it will be exclusive to the Playstation 3.
TokyoGameShow and 1UP.com said that this was false and was only a rumor.
Shack News also mentioned the game would also be featured on the Xbox360 considering that Final Fantasy XIII was featured on both Playstation3 and Xbox 360.
Even Metro Noted that it may even be released on the Wii or any other next-gen console like Wii-U

Ars Technica's Frank Caron also noted that speculation whether the gap between Kingdom Hearts 1 and Kingdom Hearts 2 would be the same. (A one year gap)(In Story-line)

What To Expect?
There's a lot of fans thinking of what they could expect but  IGN's Ryan Clements (a KH Fandom) wanted the third installment to show more romantic relationship between Sora and Kairi , Darker Antagonist and a better Combat System.
Another thing to expect would be the introduction to Marvel Characters in the series as Disney bought out Marvel Entertainment.
So there was some speculation whether marvel character's as well as Marvel Themed worlds in the newer installment. But that's just something to look forward too.

Kingdom Hearts 3 another game that takes too long to be released.

Who knows it may be released together with  Diablo III and Half-Life 2: Episode 3.
(Diablo III and Half-Life2:Ep3 are both long awaited titles to be released)"

According to some sites, Should be out by June 16, Though no fixed date has been confirmed yet.

This post on the 30th may
Vine Report

Earlier last year they had a survey asking what the fans would want in the game, (March 2014)
Survey

So will 2015 finally be the year for Kingdom Hearts 3 on the PS4 and Xbone, probably YES.
Some characters are also rumored to make a cameo appearance on the franchise
Captain America, Queen Elsa, Darth Vader, Princess from Tangled and even some characters from "Big Hero 6". Now wouldn't those be some interesting summons,

Maybe a Star Wars or Marvel themed world, Would there be more development between Sora and Kairi.
Really can't wait for the release.