The only semi-fun world to explore in KH3 was The Caribbean, and even then it had the ship battles which are annoying. Everything else is just a bunch of the same corridors/empty fields from KH2 and DDD, just disguised as fully-fleshed out worlds because they have NPCs sometimes. "Oh, they're so big, so they're better," is a ridiculous statement to me.
And yeah, KH1 is the only one where they actually tried with the world-design, what with the platforming and puzzles, and shit. It incentivized exploration and backtracking. After that, you just run forward until the next cutscene plays.