Why do the "Three Musketeers carry swords and not muskets?

The muskets were primarily a combat weapon fro large scale battles. They were well trained in their use and in tims of war were quite capable of using them with deadly accuracy. The sword remained throughout the 1700's and even into the 1800's to be the main weaopon of personal protection and single combat. Once firearms that were completely self contained, could fire multiple shots per load, and could be reloaded quickly came into being, the sword all but vanished in that capacity.