I mean, read the books… the reason he was rejected from the musketeers was because he needed to serve in another unit for a period of time. he goes away getting into duels with everyone, because he was a gascon, and they were basically the muricans of the time. In any case, during his adventure in which he was doing a favor for the queen

during the ordeal, the Cardinal’s man- Count Stinky Cheese- (aka Captain Balthasar de Montmorency, Count of Rochefort,)- was given a full parden for things he did. D’Artagnan got caught and was being accused of treason, but had successfully gotten the favor done. Since he was apparently Capable, the Cardinal handed him a captaincy with his soldiers; in part for averting the worst of the Cardinal’s plans. (I think the king suggested it as a way to get the D’Artagnan the experience for the musketeers.)