Early in the book, d'Artagnan is given gifts by his father and mother as he sets off for Paris to join the King's Musketeers. His father gives him advice to never back down from a challenge or a fight, and always be ready to jump head first into an adventure for glory. His mother gives him a recipe for a nigh-miraculous healing salve, and the narration stops to note that he's especially going to need it if he follows his father's advice.
D'Artagnan's introduction to the Three Musketeers. On his very first day in Paris he has managed to offend and make an appointment of honor with all three of them. Athos is first up and naturally he has invited the other two to be his seconds. The ensuing scene is totally hilarious from D'Artagnan's dry "your friendship is not founded upon contrasts". Porthos and Aramis immediately protest;
Porthos: "But this is the man I am going to fight!"
D'Artagnan: "But not until one o'clock",
Aramis: "But this is the man I am going to fight!:
D'Artagnan: "But not until two o'clock."
Athos betrays understandable curiousity as to how D'Artagnan has offended his friends. Both Porthos and Aramis lie and the quick witted D'Artagnan backs them up. The long sequence ends with D'Artagnan offering his apologies to Porthos and Aramis - since Athos has the right to kill him first - then drawing his sword with a flourish, "And so, gentlemen, my apologies and en garde!'
Mousqueton's story about his father the highwayman is darkly funny. During the war between the Huguenots and the Catholics, Mousqueton's father practiced both religions—when he saw a Catholic walking down the road, he would be filled with protestant zeal and rob the man. When he saw a Huguenot, he'd suddenly be overcome with such faith in the Roman Catholic church that he would be compelled to attack the Huguenot. This continued for some time, until both a Catholic and a Huguenot he'd robbed before came upon him at the same time, put aside their religious difference, and hanged him from a tree. They then boasted about this in a tavern, and were overheard by Mousqueton and his brother—and their father, in his great foresight, raised Mousqueton Catholic and his brother Huguenot, so that after they left the tavern, each brother followed the man opposite their faith and returned the favor.
The entire scene of D'Artagnan coming across Aramis all ready to take the Jesuit robes, having written his thesis and trying to prepare his soul. From D'Artagnan's agony at the Latin and theology flying around, to his Get a Hold of Yourself, Man! speech, which soon morphs into "I guess you don't need this letter, then," and Aramis' reaction ("Take away these horrid vegetables"), priceless.
Porthos' method of "arranging" duels. And later, his use of said method on Saint-Aignan, in which he casually destroys a chair.
Athos and Grimaud in a wine cellar...for two weeks. The best part was when D'Artagnan managed to get them out.
"Some wine!" said Athos, on perceiving the host.
"Some wine!" cried the stupefied host, "some wine? Why you have drunk more than a hundred pistoles' worth! I am a ruined man, lost, destroyed!"
"Bah," said Athos, "we were always dry."
The Jail Bake in Twenty Years After. After receiving it the Duke of Beaufort invites his jailer to dinner, tells him of one of his many methods to break out describing the content of the Jail Bake, and proceeds to capture and tie him up before breaking out.
Porthos causes a lot of these, from his prodigious strength to his tendency to speak his mind regardless of how appropriate it is.
After being told not to mention the name Mordaunt in front of the English soldiers: "But why? I'm speaking French and they're English." D'Artagnan just stares at him.
Mazarin is forced to give Athos his sword, telling him to keep it as a souvenir, Athos promises to keep it on him at all times. D'Artagnan declares himself moved by the exchange of promises.