The third film or book in a series — the one that comes after the sequel. It is a joke word, made by squashing 'three' into 'sequel', and everyone who uses it knows it is a joke; that playful feeling never wears off. So the pattern runs: the original, then the sequel, then the threequel. People use it in film reviews, entertainment news, and casual talk online. It often carries a slight sigh with it, because third films have a reputation for being weaker than the first two — calling something a threequel can quietly suggest the studio is stretching the story to make more money. Not always, but the flavour is often there. It is informal. Do not put it in an essay or a report. In neutral writing you would say 'the third film in the series' or 'the third instalment'. You may also meet 'prequel' — a story set BEFORE the original — which is a much more established word and can be used with a straight face. 'Threequel' is still the odd one out.

第三集（戲稱）。是玩笑造出來的字，帶點調侃，暗示第三集常常不如前兩集。正式寫作要說「系列第三部」。