Rephrasing those sentences, I get the following:
1. In my opinion, no one will buy a book written by a lunatic
2. In my opinion, no one will buy a book that they believe to be written by a lunatic.
3. In my opinion, no one will buy a book written by a person they believe to be a lunatic.
The only sentence that is significantly different is 1. 2 and 3 are essentially the same. 3 would be different if the phrase "a person they believe to be a lunatic" were changed to "a person, whom they believe to be a lunatic." In that revision, the noun doesn't have an adjectival clause attached to it. That is, the fact that the person is a lunatic is not essential to understanding that person's identity. So the sentence would mean something like "In my opinion, no one will buy a book written by a person, whom they believe to be a lunatic."
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