I've seen this before 2004. I am 22, and I remember first seeing it when I was 8 or 9. This is tripping me out! I can't find any references as to why I knew this movie before it came out in 2004.
Sorry, but you must be thinking of something else -- like Mickey in Prince and the Pauper maybe. The idea of Mickey in Three Musketeers has been kicking around since the late 40s but this is the only time it got produced.
Like the original poster, I could have sworn that I'd seen this before, or at least the swashbuckling parts. I don't remember the framing story of a turtle who gets to narrate his comic book thru a mischance, but the evil Captain and the musketeers-by-mistake plot sounded and looked awfully familiar.
Is is possible this started out as a short that was then expanded into a full-length cartoon?
I remember seeing this movie sometimes in June, 2004 even thought the DVD release was on August, 2004. Perhaps my memory is fuzzy. --- Talking about my movie experience in here: http://astarisborn94.livejournal.com/