With regard to an essay about Harriet Ross Tubman, please give three important main topics with examples.
I have not read this book (or if I did it was decades ago), but I see that your question is still unanswered. Harriet Tubman is a relatively familiar character, so I'll offer you some ideas for writing an essay about her which could also apply to a speech. You can then do whatever research is necessary and apply it directly to your writing and speaking.
An essay is generally a discussion of three key components or points about whatever topic you've chosen. In the case of a person, those points can be decided various ways, all of them logical and easy to determine.
The first option is a chronology. Divide her life into three segments and use them to discuss the relevant moments in her life. For example, you could begin with her birth and go until she begins her work with the Underground Railroad and finish with the end of her life. One more strategy is to break her life into years, based on what she was doing at those times. A chronology is fairly clear-cut but may not meet the needs of your presentation.
Another possibility would be to select three aspects of her life and deal with those--perhaps her life, her legacy, her work, her family. Since you're more familiar with her story, you can certainly find some options you feel comfortable with and can use.
Whatever your three points, be sure to find a good example, story, or quote from her life to catch your audience's attention in the introduction. Have a strong, one-sentence purpose (thesis) statement which encapsulates your three points, and be sure to review thosethree points in the conclusion, as well. The eNotes sites I've included below, particularly the first one, might also give you some ideas to incorporate in your speech and/or essay. Best of luck!