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!
Rahim is a man who likes scholarly things. He recognizes in Amir that Amir has a gift in writing. Unlike Amir's father, he is able to identify Amir's strengths. Baba seems disappointed in Amir. He feels that his son lacks courage. He does not care about Amir's talents. When Amir studies the Koran under the guidance of the religious scholar his father jumps Amir's case when he tries to share what he ahs learned. Later we learn that his father was right to be concerned about the religious Zealots, but the way he relates to his son is negative and condescending. Amir never seems able to please him until he wins the kite contest.
One example of support and guidance that Rahim gives Amir as well as comfort is during Amir's birthday party. Amir is upset after having had to take a gift from Assef, his enemy. His father had been embarassed by Amir's behavior. Amir goes off to be alone. He comes across Rahim in the garden. In Amir's mind he narrates;
"I'd always thought of him as Baba's quiet alter ego, my writing mentor, my pal, the one who never forgot to bring me a souvenir,"(98)
He is sharing the story with Amir that he had once been in love with a Hazara girl, the ethinic group that is considered to be lesser. When he had told his father his father sent her and her family far away. As he finishes the conversation he tells Amir that he can tell him anything. Amir contemplates telling him about the incident of the rape and his fear of Assef, but he can't. Rahim is able to identify that something is troubling the boy, when Baba does not seem to see anything but his own needs. Even the leather bound notebook that is a gift from Rahim reflects their bond and understanding of one another.
When Amir goes to see Rahim after the death of Baba and the letter from Rahim, he tells Amir that he always knew he would be a writer. He learns Rahim is dying and asks him to go home with him so he can get him a doctor. Amir wants to cry. (Chapter 15)
It is Rahim who finally tells Amir the truth about his half brother Hassan. It should have been Baba's responsibility, but in the end it fell to Rahim.
In many ways Rahim had been a father to both boys, Hassan and Rahim as he had continued to look out for Hassan after Baba and Amir had left. Even in his present actions he is serving as a father trying to unite two sons.