Actually no. The first book is how d'Artagnan met the Three Musketeers. Although he was with the Three in the story he was not one of the three, or they would have been called the four musketeers. It was the three musketeers plus d'Artagnan.
Its like calling Brian Epstein the Fifth Beatle, because he was not one of the "Fab Four".
So there.
I said a brotherhood of 4; not that D'Artagnan was a musketeer... So there...