It started with Aristotle’s Poetic with its beginning, middle and the end. Note that we have the three stooges, three little pigs, three musketeers, the holy trinity, etc. The porridge was too hot, too cold and just right with papa bear, mama bear and baby bear. The Garden of Eden had three players – Adam, Eve and the snake.
The Rule of Three is an excellent rule. I wish more presenters would make use of it. Many of them feel that they need to tell the entire history of the western civilization in 40 minutes or less. Engineers and scientists are notorious for that because they fear that the audience will fault them for “leaving something out”. So instead of conveying three main points in a presentation, they jam everything they know onto a few slides with those dreadful bullet points and expect the audience to digest it all.
So focus on three main points in your presentation.
The first picture shows what happens when you put the small pebbles in first. The larger pebbles cannot get inside the jar.
Now if you were to put the large pebbles in first and then fill the void with the smaller pebbles, you can fit them ALL inside the jar as shown in the second photo!