Three Musketeer

Etymology 1
Formed due to a misinterpretation of Three Musketeers as a compound noun that includes the stem three rather than as a plurale tantum proper noun.

Noun

 * 1)  Any one of  in ' novel of the same name, i.e. either, , or.
 * Is d'Artagnan a Three Musketeer?

Usage notes
The proper usage would be to say "one of the Three Musketeers" rather than "a Three Musketeer." Or, rather than "two Three Musketeers," the proper usage would be "two of the Three Musketeers."

Etymology 2
See.