When reading the title of the book, we don't know what three men it is talking about so there is no/indefinite article (the same with "a boat"). But why is there "the Dog" instead of "a Dog"? We don't know anything about the dog either.
Update: Thanks for the answers. However, I'm not sure why the question got down-voted. I'm not a native speaker and my language has no articles, so using them properly and knowing different corner cases in English poses a problem for me. Honestly, I'd like to know the reasons for down-votes so that I could ask better/more qualified questions in the future. So I kindly ask people who down-voted to explain the reasons.