I figure the title is meant to be interpreted as "D'Artagnan and the Three Musketeers". He's the main character and the book is about his adventures with them. What does bother me is the total lack of muskets.
I'm not sure where you get your information but I'm pretty sure their names are Mickey Mouse, Donald Duck and Goofy. Pluto was just a dog in that movie.
It's about D'Artagnan's journey to becoming a Musketeer, so calling it 4 Musketeers would be Spoilers! A better choice would be "The 3 Musketeers and I" which my old brain is hinting may have been the title of some film version...
Because he wasn't actually a musketeer (not until the very end anyway). He was friends with the musketeers, but was basically just a regular guy off the street. A musketeer was some kind of position in the military/royal guard or whatever. He didn't hold that position.
Ah, I haven't read the other two. I actually didn't even know they existed until you told me just now. But for the first book he's definitely just a friend of the Musketeers until the very end.