Why did they call them the "3 musketeers" when they used swords, not muskets?

Because the novel by Dumas is based on Mémoires de Monsieur d’Artagnan, capitaine lieutenant de la première compagnie des Mousquetaires du Roi (Memoirs of Mister d’Artagnan, Lieutenant Captain of the first company of the King’s Musketeers) by Gatien de Courtilz de Sandras which was then based on the Musketeers of the Guard who were created when one of the King Louis’s created a company sized element of light cavalry equipped with muskets. And besides, no one reads about musket battles, so boooooooooooooooooorrrring.

Musketeers had to be proficient with both a musket and a sword because the firearms technology of the time gave the musket limited use. Musketeers always carried swords as a sidearm to be used when the musket was inappropriate for the situation. Note that the battlefield scenes in novel do feature the main characters using their muskets.

http://en.wikipedia.org/wiki/Musketeers_of_the_Guard

For the same reason, I suppose, that “infantry” is called by that name, even though none of them are infants.

@CWOTUS The term “infantry” comes from the fact that it was the youngest and least experienced people were put into that division of the military. In that sense, then, they really are the infants of the army.

Who are they?

Some French dudes. You may safely continue to ignore them.