First of all, the Musketeers were an elite French military unit charged with protecting the King. They were kind of like the Secret Service, except they could also go to war. The three Musketeers of the title are Aramis, Porthos, and Athos, three close friends and exceptional Musketeers known as the Inseparables.
Now, here’s the catch - the novel is actually about four best friends, and although D’Artagnan becomes a Musketeer only three-quarters of the way through the novel, Dumas could still have titled this work Four Musketeers without anyone calling him out on it. The novel does, after all, pertain to four Musketeers. So why three, exactly? Your guess, as usual, is as good as ours.
Here’s ours: The Three Musketeers is a title that deliberately isolates D’Artagnan, which makes sense, since he’s the protagonist and hero of the novel. And since D’Artagnan’s goal throughout the novel is to become a Musketeer, if Dumas had entitled the novel The Four Musketeers, that would have given the story away, wouldn’t it?