Simplify the following text for a Grade 8 native English-speaking person.
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 ! ) .