Why are the Three Musketeers called musketeers when they use swords (not muskets)?
Er, because they were issued, and used muskets, as well as swords? In battle, muskets were their primary weapon, not swords.
Try Google, “musketeers”. Go into Advanced Search and tell it “without the words ‘Chris O’Donnell’”, you’ll get better hits. 
Yea, the sword was the dueling and gentleman’s weapon, and the musket was the soldier’s weapon. They rarely show the 3M (if ever) in actual battle.