What is the best way to structure an essay that is asking me to analyse three art works.

There is more than one way to do this; however the method that has proved most efficient and effective for me is to begin with an introduction that briefly addresses the question (by providing some contextual background) and which then outlines the title, date and artist of the three works you will be analysing.  Then, address each artwork in an orderly fashion in three different paragraphs. In each paragraph, follow the same structure. In other words, first begin by providing a bit of basic background information concerning the painting, and briefly outline its composition. Then, address exactly what the question is asking (make sure to focus on the key terms used in the question, and use these same words, or close synonyms when analysing your artworks). Make sure to be specific in your visual analysis. For example, if the question asks you to talk about the style of a painting, make sure to describe a specific area in the painting where this is apparent. For example, if the painting can be classified as an Impressionistic painting, talk about a specific area in the painting where one can clearly observe thick and unblended brushstrokes and the effect this creates.  In the conclusion, make a concluding remark that highlights a common aspect in all three paintings, and maybe something that makes one or two of them stand out from the others.