Organizing your essay into sections which each have their own focus will help you get to three pages. For instance, you can write one section on why the book was banned in Mississippi and dedicate three paragraphs to that subject.
Another three paragraphs can be dedicated to another example of where (and wh)y the book was banned.
Allowing for several paragraphs for introduction, transitions, and conclusion, you will only need three or four sub-sections in your essay to reach three pages, so reaching the desired minimum won't be a problem.
If you select a number of examples to address in your essay and analyze those examples first, coming up with a connecting thesis can be your next step.