Most instructors dictate that you have at least three points of defense and/or explanation. I assume three was chosen because it is the number of the trinity or because it was inspired by Hitler's Third Reich. The three paragraph essay is stifling to creative expression in its rigid structure. An English teacher once tried to explain the divinity of the three paragraph essay by comparing it to a triangle saying, "The triangle is one of the strongest structural shapes, which can endure a great amount of weight that's what makes it perfect for an essay." I personally don't appreciate the comparison, because in my opinion the triangle is the most dangerous shape. It has pointy ends which can break skin depending on the degree of the angle, but that's geometry. It has also been brought to my attention that a three paragraph essay looks like a fat chick. A small light bit of writing on top followed by a heavy middle section ending with a little bubble of words for feet. It just isn't proportional or aesthetically pleasing. The three paragraph essay also doesn't lend itself to an agreeable flow of thoughts. Everything of significance is crammed into the center. Three paragraph essays read more like an E. E. Cummings poem whereeverythingisranintoexaggeratetherateofspeechorthought. Jokes aside, it really does push everything in to close together. Readers that skim through derive very little because there are no breaks between the thoughts to slow them down to take matters into consideration. New ideas should not just be broken down into sentences they can be expanded into paragraphs so one has more room to explain. Having only one paragraph pressures writers to use concise words, and let's face it some of us don't like to visit words outside of: good, bad, cool, dumb, and weird.
The three paragraph essay of course cannot do without the conclusion. So, this gives the writer an opportunity to be redundant by saying what they have already wasted our time with. Three paragraph essays are rigid, terribly proportioned, and dull to look at and read. That's why writing three paragraph essays are annoying to write.