The infamous 3rd entry in a trilogy is so often a disappointment. It's pretty much an accepted cinematic superstition at this point that the third movie is bound to derail a franchise. Return of the King, Toy Story 3 and Raiders of the Lost Ark are rare exceptions to the rule. For every one of these successful third installments, there's many more that leave a sour taste in fans' mouth.
So, which one was the worst?
My vote goes to RoboCop 3. Everything about it is an unmitigated disaster. That movie by its very existence should be the measuring stick by which
not to end a trilogy (or any installment in a franchise). It was so awful, Nancy Allen literally begged to have her character (Anne Lewis) killed off after reading the script, and I honestly don't blame her. Above all else, it commits the cardinal sin:
dumbing-down RoboCop.