There are many Dukes of Buckingham in history; the one in The Three Musketeers is George Villiers. He is an actual historical figure who was assassinated by John Felton before he could send aid to the Rochellais.
In the novel, Buckingham is portrayed as a gentleman of the highest order who is obsessed with Queen Anne. In England, he holds a position similar to that of the Cardinal in France. The two men both serve weak monarchs, and are the true administrators of their respective nations. The Duke has no problem abusing his power and wealth, however, to try and win Anne’s love.