The Three Musketeers is an 1844 novel by Alexandre Dumas. The story involves D'Artagnan becoming one of the musketeers.
In the episode "The Short, Quick Summer," Arthur remembers that he read the novel over the summer. He then imagines a scene from it, replacing the usual characters with his friends. It is also partially mentioned in "What Scared Sue Ellen?" when Arthur says "all for one", but Binky adds "and good for nothing".