No, not every essay will have three body paragraphs. It depends on the topic and how much information is needed to support the main idea. Sometimes an essay will only have two body paragraphs, or even just one. However, three is typically the minimum number needed in order to sufficiently develop an argument. Therefore, if you are unsure how many body paragraphs to include, it is usually safest to err on the side of including more rather than less. This way, you can be sure that your argument is sufficiently developed and supported.