Tora, I am not quite sure about what did you mean. My question is, are you going to select one of those topic sentences or elaborate it to become 3 paragraphs for each topic sentence? Perhaps the thing that I would able to help is to rephrase your topic sentences.
Here you are:
Christopher doesn't show signs of change but he becomes more independent as he faces difficult challenges.
- Difficult challenges lead Christopher to be more independent even though he does not indicate any signs of alteration.
Father grows as a person as he accepts his mistake and commits to rebuilding his relationship with his son.
- By re-establishing his relationship with his son, father is able to grow become a person who accepts his past mistake.
From the beginning, mother was introduced as hot-tempered who ...
- Since the beginning, Christopher's behavior becomes a problem which his mother could not deal due to her hot-tempered character, but then eventually she becomes an independent woman and learns to cope with Christopher at the end of the novel.
As you can see, I did some paraphrase towards you topic sentences. I hope it helps you.
Good luck for you assignment! :)