Despite being a work of fiction, quite a few. The four musketeers were all real: D'Artagnan is based on Charles de Batz de Castlemore, Comte d'Artagnan. Athos was Armand Sillegue d'Athos d'Auteville. Aramis was Henri d'Aramtiz, and Porthos was Isaac de Portau. Louis XIII, Anne of Austria, Cardinal Richelieu, and the Duke of Buckingham were all real characters, and the story of the diamonds is an actualy incident. Charles de Batz de Castlemore was actually too young to be involved, but d'Athos, Aramitz and Portau were all involved in helping the Queen recover her diamonds, while Lucy Hay (nee Percy), a lady in waiting of the Royal Court, was instrumental in giving Richelieu information about the Queen's love affair and the diamonds that were given away. Hay became the basis for the Countess de Winter. Constance and Monsieur Bonancieux do not appear in the historical record, and de Winter's past with Athos, and her vengeance against the Musketeers, are inventions of Dumas.
Georges Villiers, Second Duke of Buckingham, was also Prime Minister of England under Charles II, and therefore also in a good position to travel regularly to France and authorize military expeditions, as he does in the second half of the film.