Friendship
The overarching theme of novel that unifies and unites all the myriad plots and storylines is the friendship between the musketeers. Once D'Artagnan has been accepted as one of the Musketeers, they never argue or engage in disagreements. The tension here is not between the musketeers, but between the musketeers and their enemies and foes. The musketeers are also unfailingly loyal to one another, and willing to help each other with any challenge or task they take on. Friendship also involves trust: while they may not always be totally transparent with one another, they always assume mutual goodwill. While the friendship is intense, it is also related to a specific life stage when the men have relatively few responsibilities. By the end of the novel, the musketeers have reached a point where their lives are going in different directions, and D'Artagnan mournfully predicts that it means friendship will no longer be at the center of their lives.
Class
The Musketeers are in service of the King. This is not a novel about rebellion or revolution; in fact, The Three Musketeers is a profoundly conservative novel when it comes to the division of classes and the social hierarchy such divisions create. As the Musketeers go about their business of protecting the interests of the aristocracy, the place where each person fits within the system is clearly delineated and never questioned. It is a system designed for order and security and such ideals as personal liberty or allowing an individual to rise to a higher station are not only never considered, but actively rejected as a threat to the system.
Honor
Personal honor is considered extremely important among many of the characters, especially the musketeers. Protecting one's reputation for being brave, loyal, and chivalrous is very important in a world where reputation could determine opportunities and access to power. Because they are hot-headed and rash, D'Artagnan and the musketeers can seem almost comical in their tendency to quickly rush to defend their honor at the slightest possible insult. At the same time, a code of honor also creates respect even between enemies. The Cardinal gradually grows to admire D'Artagnan because of the way the young man conducts himself, and D'Artagnan can notice when an opponent is a worthy adversary.
Vengeance
Much of the action of the novel is driven by the single-minded desire to enact vengeance. Milady seeks revenge on the Comte by seducing D'Artagnan, only to later seek revenge on D'Artagnan by killing Madame Bonacieux. Much of the Cardinal's initial scheming is driven by his desire to have revenge on Queen Anne after she rejected him. John Felton kills the Duke of Buckingham as revenge for the crimes he believes Buckingham committed against Milady. The eventual death of Madame Bonacieux is presented as justice, but is also a form of vengeance because it happens secretly and without any legal or official trial. Vengeance is shown to be a powerful motivator of human behavior, and also a sign of a world where individuals typically act in their own best interest and do whatever is necessary to maintain power.
Secrets
The theme of secrets is part of what makes the plot of the novel so dramatic and compelling. Beginning with D'Artagnan's mysterious encounter in the town of Meung, things keep happening which seem to have hidden meaning and be connected to each other. Because of the elaborate web of deceptions spun by the Cardinal and Milady, events and people always seem to carry more significance. Secrets are part of why every choice is dangerous and high stakes in the world of the French court; it is impossible to ever know the full story, and what the ripples of an action might end up being. Secrets also mean that the past can never be fully put away. For example, Athos tries to keep his disastrous marriage a secret, but it eventually comes back to haunt him.
Seduction
Seduction is a powerful theme in the novel because it leads to otherwise intelligent characters making flawed choices. Queen Anne and Buckingham are seduced into a dangerous affair with high stakes and political consequences because of their loneliness and attraction to one another. Even though he is cautioned by his friends, and suspects her to be dangerous, D'Artagnan is seduced by the beauty and allure of Milady. Milady, in particular, wields seduction as a weapon which allows her to achieve her goals: she is able to gradually win John Felton over, and eventually persuade him to liberate her and kill Buckingham on her behalf. Seduction is a key strategy because it involves a great deal of cunning and psychological awareness. Milady is seductive not only because she is beautiful, but also because she knows how to find and exploit the vulnerability of others.
Growing Up
At the start of the novel, D'Artagnan is a young man who is trying to find an identity and place for himself in the world. Because he is a nobody, he has to make his own reputation and earn the respect of others. D'Artagnan also has to decide what kind of man he is, and what values he wants to live by. Once he finds support and mentorship through his surrogate family of Treville and the Musketeers, D'Artagnan develops a strong sense of identity, and never wavers in his commitment and loyalty. While D'Artagnan occasionally makes mistakes because of his naivety and youthful recklessness, he gradually matures and proves himself worthy to be a Musketeer.
