I don't like the idea of a game just to cover what happened in the series for people who didn't play all the games.
I think a more ideal idea would be to have a small "film" of some sorts included in KH3 that you can view before you play the game. Like a "Story Thus Far" thing. You could argue that the opening would be that, but I'm thinking something more detailed.
That would work too, like the movie in FF VII COMPLETE