It’s one of those things where we shortened a word and then it stopped making logical sense.

“The three musketeers” werent just musketeers who carried muskets.

They were “the king’s musketeers”. They were elite special forces as well as the personal bodyguard for the King. The best of the best. The “musketeer” part was the common bit, it just sounds fancy centuries later.

But the book might as well be called “The Kingsguard”