Try the third. It's by far the most experimental (pretty tame and partially digested experiment by jazz standards, but not your average blockbuster rock album by any means). I don't think it did as well as the first two which might explain why they started to draw in their horns (so to speak!).
II and III were bought in my first year of record buying - I always loved the sheer range of the albums. Good songs, range of styles (everything from pop to blues to country to dipping a toe into freer worlds [but not for more than a minute or so!]) and marvellous brass arrangements.
At the time I had no context for this music outside of rock; but listening to some of the New York Latin/Boogaloo music in recent years I can see where some of it came from, especially the horns.