But like, only trilogies. None of this "Here's the first three books... and there's four more!" You ever see when someone calls the first three Harry Potter books a "trilogy"? What is wrong with those people?
Yes, some of these trilogies have prequels and spin-offs and sidequels and one-offs, but they primarily function as trilogies. 1, 2, 3. Trilogy.
Well, there is a trilogy of five, but that's a special case!