Okay, so I don't know what started off the whole trilogy thing. Maybe it was the Godfather, or the Dollars trilogy. Answers vary, some even dating as far back as Boris Karloff's stint as Frankenstein in Frankenstein(1931), Bride of Frankenstein(1935), and Son of Frankenstein(1939). I couldn't say for sure, of course - I haven't seen enough films to say for certain, and even if I had my opinion is a drop in the film-review ocean - but what I do know is this:
I utterly hate them.