The Three Musketeers - In plain text, or as a zip file, at Project Gutenberg. - http://www.gutenberg.org/etext/1257
Literature Network: The Three Musketeers - Chapter indexed HTML of the complete text. Includes a search feature and an author biography. - http://www.online-literature.com/dumas/threemusketeers/
eBooks Cube: The Three Musketeers - Chapter indexed HTML of the complete text. Includes author information. - http://www.ebooks3.com/ebooks/the_three_musketeers.html