Differences from the book
First of all, I think that all changes were made so the movie ended more polished and had a with a better flow. I just want to point some of them.
On the first duel, Dartangnan:
book: kills Jussac, who is a great soldier
movie: Jussac re-appears at the ending for a more dramatic climax
with the Bonacieux:
book: monsieur bonacieux ask dartagnan to look for his kidnaped woman
movie: dartagnan meets miss bonacieux right away and have sex with her
the queen:
book: gives diamond needles
movie: gives a diamond collar and a big combat happens
travelling to londom (biggest change):
book: every musketeer falls on a different place, dartagnan make it to londom after a fight with Wardes. Dartangan goes looking for them AFTER returning the needles to the queen.
movie: the three musketeers are attacked on the same place, dartangan fighs Rochefort, Planchet help the three to join Dartagnan at the queen's party.
also; Grimaud, Musqueton and Bazin are absent from the movie (the worst change)