What do you think about the author's point? - Say "point of view" or "viewpoint" and then the sentence is correct. You could say "What do you think about the point the author made?"
What do you think of the author's point? < The same answer applies here as above. You can say "think about" or "think of". Both are correct.
How do you think about the author's point?< Use "what" not "how" for this sentence.