What is the theme of the book The Three Musketeers, by Alexandre Dumas?
There is not one main theme, but here are a few which are recognized as major themes in the book:
"The Quest": the quest of a young man to make his fortune
"Brotherhood": the camaraderie of a group
"love": what true love means, and how to express it-- the Muskateers believe in chivalry and honor