Cargo Ship: Twenty Years After reveals that as a teenager, Athos had a crush on a Greek statue.
Complete Monster: The mysterious, murderous Milady de Winter is one of the top agents of the visionary Cardinal Richelieu in his campaign to strengthen France and free it of foreign influence, but shares none of her employer's lofty ideals. Instead, she uses her position, resources, and quasi-supernatural beauty and charisma to indulge her limitless appetites for money, power, and indiscriminate, disproportionate revenge on anyone who gets on her bad side. Ever since her humble origins as a larcenous nun, she's targeted and seduced any sufficiently-useful man she comes across, destroying those who reject her and either killing or abandoning anyone unlucky enough to become her thrall as soon as they're no longer valuable. Her steadily-escalating battle with d'Artagnan brings out all her worst excesses, as she first tries to enlist him to kill her brother-in-law for his inheritance and a young noble for apparently turning her advances down, then, after he humiliates her, repeatedly attempts to kill him with zero regard for collateral damage, endangering and killing several innocents along the way, and finally murders his Love Interest as Revenge by Proxy whilst pretending to be her dearest friend.
Common Knowledge: Many people point that the three Musketeers are actually a group of four persons. While this is indeed the story of four men, this remark forgets that d'Artagnan only becomes a musketeer halfway through the novel, as a reward for the exploits at the Bastion of St. Gervais (the Cardinal offers to have three fleur-de-lis embroidered on the famous napkin-flag for Treville to use as a company standard, who points out that this isn't fair since d'Artagnan is a guard, not a musketeer. "Take him, then," the Cardinal says.).
Designated Hero: Due to some intentional Anti-Hero traits and a lot of Values Dissonance, both deliberate and not, our musketeers can come across as real shit-heels at times. To modern eyes, their dueling at the drop of a hat would be considered near psychopathic, and most of their romantic endeavors constitute sexual predation. Some of this is simply the fact that the stories were written in the 19th century, but a lot of it is the intentional lionizing of The Cavalier Years, when the stories will have you believe that men fought, drank and loved harder than any modern sissies would dream. Part of the fun is following these larger-than-life characters through adventures that excite as well as scandalize.
Ensemble Dark Horse: Rochefort, and the servants Planchet and Grimaud, who take a level of badass between the first and second books. Also, the Duke of Beaufort.
First Installment Wins: Most people only know of The Three Musketeers as characters, unaware that they star in a series of novels as opposed to one book. Even for those who are familiar with the books, the first is by far the most popular, with The Man in the Iron Mask being perhaps second-best known among their various adventures.
Magnificent Bastard: Cardinal Armand Richelieu is the most powerful man in France, and the ruler behind the throne. Seeking the betterment of France as a nation under his guiding iron hand, Richelieu schemes to strengthen the monarchy and to also start a war with England to further check Spain and Austria. In order to disgrace his rival, Queen Anne, Richelieu convinces the king to throw a party and request Anne wear diamond studs he gave her as a gift, well aware Anne has given them to her lover, the Duke of Buckingham, which will discredit Anne and begin a war with England. When the Musketeers recover the diamonds in time, Richelieu accepts it with grace, later deciding to have Buckingham assassinated and presenting the wicked Milady de Winter with a letter excusing her from all acts she commits in service to France. When Milady is executed by the Musketeers, young hero D'Artagnan thinks to save himself by presenting Richelieu with the same letter, only for Richelieu to display his own power by tearing it up. Impressed by D'Artagnan, however, Richelieu accepts him as a Worthy Opponent and a boon to France, writing him an officer's commission to the Musketeers before focusing on his next schemes to ever better France as a nation.
Tear Jerker: In The Man in the Iron Mask, although it tells of the deaths of Porthos, Athos, and d'Artagnan, and although they are all tragic in their own ways, it was really the noble sacrifice of the lovably naive and childlike Porthos.
D'Artagnan pulls a Bed Trick on Milady, which by modern standards is definitely rape. While it's hard to argue that Milady is an innocent victim (she did intend to kill the man that she was originally going to sleep with), it was hardly necessary and does no favours for his audience sympathy.
Frequently lampshaded in other cases by Dumas, as he often breaks the narrative to wryly note that his heroes' womanizing ways were just common practice in those days. Possibly actually meant to be a Take That! against practices in his own time. This trope is notably averted when D'Artagnan seems just as troubled by Athos' murder confession as the reader is. The fact that he later turned out to be mistaken only complicates the matter further.
Villain Decay: Going along with Le Vicomte de Bragelonne's main theme of how the new generation is far too Romantic and not as adventurous as the previous one, the villains suffer as well. De Wardes (son of the man d'Artagnan wounded in the first book) stirs up some romantic tension and briefly duels two people, but beyond that, is a far cry from Milady or Mordaunt. Similarly, Colbert lacks the respective cunning and manipulation of Richelieu and Mazarin.
What an Idiot!: D'Artagnan, naked and in bed with Milady, thinks it's a good idea to inform her that the reason for de Wardes not answering her letters (the entire reason Milady wanted him dead and slept with d'Artagnan so he'd kill de Wardes) was because he'd intercepted her first letter, then passed himself off as de Wardes in the dark.
Woolseyism: "All for one, [and] one for all" is inverted in the original French - "Un pour tous, tous pour un" - but the English order is better known (in the English-speaking world). It all comes from the Latin "Unus pro omnibus, omnes pro uno". A rare English-language adaptation which kept it as "One for all, all for one" was The Man in the Iron Mask (1998).