You have a good idea for a paper on The Kite Runner because there is a great deal of evidence to show that Rahim was a better father than Baba. But I am not certain, when you refer to three paragraphs, whether you are referring to three body paragraphs or simply a three-paragraph essay. So, I will discuss both ways of writing an essay.
If you are supposed to write a three-paragraph essay, the first paragraph should be the introduction, the second paragraph should be the body of the essay, and the third paragraph should be the conclusion. In the introduction, you need to give the name of the book and its author, a brief overview of the book, and your thesis statement, what you are trying to prove as you write, that Rahim was a better father to Amir. In the second paragraph, you need to offer examples of his being a better father, examples from the book. Finally, the conclusion is a kind of wrap up paragraph, in which you remind the reader what your thesis is and give the reader a little review of the points you made.
If you are suppposed to have a five-pararagraph essay, the structure is not all that different. The first paragraph should be the introduction. Then you will need three paragraphs that show why Rahim was a better father. Each paragraph should be about just one reason. Finally, you will need a conclusion.
Now, what kind of evidence will you provide to support your thesis? I remember that Rahim gives Amir a very important gift, a writing journal, which shows that Rahim has greater insight into Amir's needs than Baba does. But there are many other examples in the story, and if you need three, for three separate paragraphs, you should have no problem finding them.
Coming up with sentences is not so difficult when you have good evidence. Just remember that you must connect everything to your thesis. You can start out your sentences by saying "Rahim was a better father to Amir because...." Once you have written about your evidence, you can always go back and give the sentences a bit more variety. The idea, though, is to get that evidence down on paper in a first draft. Once you have it on paper, you can work on "polishing" it.
Good luck to you!