Mr. Buzzkill here. The protagonist of the story, D'Artagnan was on route to Paris to join the Musketeers, however he is assaulted along the way and loses his letter of recommendation, thus failing to secure the prestigious position. He at first antagonises and later befriends the three titular musketeers: Athos, Porthos and Aramis. The three disband their crew by the end of the novel, while D'Artagnan becomes a fully fledged Musketeer, but the implication is he was the fourth member of the crew without being an official Musketeer, thus three.