To me, differentiating the 3-part story structure and 3, 4, or 5-act structure is one of time
As noted, the 3-part story structure is basic to almost all stories, whether movies, TV, books, etc. The act structure is specifically for television, which has a definite length (1/2 hr, 1-2 hrs) and placement for advertisements.
Which brings up a question: is there a good length for a web film, since it's not television? I originally wrote the pilot for New Gods
as a 1-hour script, as if for television, but decided to break it into smaller webisodes. Having the act structure made this simple; each act became a separate episode.
What are your thoughts regarding time? Have we been wired by TV and movies to consider .5, 1, or 2 hours the perfect length? Or are we becoming so ADD that we can't sit still for anything over 10 minutes?
And how does this apply to new platforms? Can you watch a full length movie on your smartphone? Or would short films work better?