A New Game Plus mode has become pretty commonplace among most game franchises. Once the players beat the main game, they can begin from the start again without having to loose all their equipment, but it also presents new challenges.
The NG+ mode is pretty much either loved or ignored, so it's sad to say that Kingdom Hearts 3 does not have a New Game Plus mode. It is currently not possible to create a NG+ save, but you can of course create a brand new save if you want to begin from the start again.
It might be possible that Square Enix will update the game with a New Game Plus mode at a later date, but nothing has been announced about this just yet. However, we can cross our fingers that they will eventually release it as part of a DLC or simply as a patch.