Original: D'Artagnan is not one of the musketeers of the title; those are his friends Athos, Porthos, and Aramis, inseparable friends who live by the motto "all for one, one for all" ("tous pour un, un pour tous").
Simplified: He and his friends Athos, Porthos, and Aramis live by the motto, "One for all, and all for one" (French: Un pour tous, et tous pour un!).