OK, there is a huge difference between a sequel and a third movie in the line made only for money. Generally the sequel is made to follow up the second. If that somehow makes money they patchwork the third together hopin to score a few mil. I think there really should be a sequel poll and a horrible movie that was only made to cash in poll(KKIII, JPIII, Rocky V, Lethal Wepon III)

Generally the sequel will have the whole cast back and seem like they are having a good time even if the movie sucks(Oceans 12) The worthless third one has half the cast back and patchworks them in to make money(American Pie III)