KH 2 also went with only 3 playable original worlds, Twilight Town, Radiant Garden and The World that never was.
The only ones strictly needed story-wise for KH III as we know so far are Castle Oblivion/Land of Departure, Keyblade Graveyard and possibly Daybreak Town in some form, although the trailers have more or less confirmed a return of Twilight Town.
Add Radiant Garden as a "hub" world yet again and we are at five original worlds.
KH 2 had 15 playable worlds including originals, so if we go by the assumption that KH III will at least have 20 playable worlds, five original worlds would still leave room for 15 Disney worlds, the number KH 2 originally had for all worlds.