First of all, you have my sympathy in losing your dad so young.
In this essay, please consider what your three body paragraphs will be about. Will you be discussing three adversities which he faced such as not being able to handle a full dose of chemo so that he had to go more often? Would one paragraph be about how he talked with the family about his death, allowing you to talk with him about goodbyes or things you would miss having him not be there? Would one paragraph be about the effect on the family of having a father with cancer or would you focus on only him?
Each of the questions I've included are to help you focus your essay so that your thesis statement contained in the introductory paragraph will introduce the reader to the three ideas you plan to discuss in your essay. For example, your thesis statement could be something like, 'My father, the center of our family, was diagnosed with cancer which forced him to face the effects of chemo on his body, affected the rest of the family in positive or negative ways, and because he could face his death and discuss it, so also could the family talk with him about saying goodbye.'
Just be sure that whatever you choose for your three body paragraphs, that the thesis statement introduces the reader to what you will discuss. Good luck on a difficult essay topic.