For lack of a better term, I mean entire anime side to be FF. I always considered the game to basically be a FF game with original characters + disney.There's no Final Fantasy side in KH3 at all though.
And before that, the FF element was pretty minor (i.e. Here's some cameos by past FF characters, and most of them are from this one place - Radiant Garden.)
Now, if you're equating KH's grand story to FF, that's a fallacy. KH =/= FF.