The characters in this novel come from a number of different class backgrounds ranging from servant to king, but nobody disputes their place in the social hierarchy. Rather, they all understand the limitations that class places on their ambition and work within that framework. Also, being a member of the royalty does not necessarily correlate with power. In the novel, members of the monarchy are portrayed as figureheads while real power rests elsewhere.
In The Three Musketeers, there is no correlation between class and money. You can be dirt poor but still belong to the upper class, while you could conversely be very wealthy but not considered as one of the elite.