Before Kingdom Hearts 3 was announced at E3 back in 2013, players had been waiting years for a main series entry. Now, almost a year and a half later, we're still waiting for the game. This time, the wait feels a bit more hopeful, as there is a release window that basically encompasses the whole year of 2015. Given game development's penchant for delay, we're betting that the game comes out in late 2015 or early 2016. Though, with Tetsuya Nomura recently leaving work on Final Fantasy XV, he's got a lot more free time to work on Kingdom Hearts 3.
One interesting piece of information made available recently is that Nomura and his team are working closely with Epic Games. You might be wondering why the two developers are working with one another, and the answer comes with another revelation: Kingdom Hearts 3 will be using Unreal Engine 4. The Unreal Engine is developed by Epic Games. This means that the game will no longer be using Square Enix's proprietary Luminous Engine. While facts aren't clear on just why the switch was made, you can't blame Square Enix for wanting to play it safe, going with a trustworthy name like Unreal Engine. Considering how new the Engine's fourth iteration is, we're very interested to see how Kingdom Hearts' development turns out.
Nomura has indicated that because of the switch, Kingdom Hearts 3 is currently experiencing some rendering issues. The problems don't seem to be severe, but we're going to keep listening for anymore information on the subject.
Source: Nova Crystallis