Kingdom Hearts III is being developed on Unreal Engine 4
Kingdom Hearts III will not be using Square Enix’s Luminous Engine as previously announced. Instead, development continues now on Epic’s Unreal Engine 4 director Nomura confirmed to Famitsu. In the full interview, which largely focused on Kingdom Hearts HD 2.5 ReMIX, Nomura said that development on Kingdom Hearts III was moving along as planned based on their schedule, however, the game has switched over to Unreal Engine 4 for various reasons unknown.
Because of the switch from Luminous to Unreal, Nomura says they are currently having rendering issues, although they are closely working together with Epic Games to iron out those problems.