Threequel means the third in a series, and I saw it used in the Radio Times (a respectable publication) with reference to films.
I mean, I've only just got hardened to the horrors of flipping prequel.
Word Not To Use Today: threequel. This word...it's like viewing the bleeding corpse of some new Frankenstein's monster. The word is formed, presumably, under the illusion that the se in sequel means following and quel means next (or item, or something)...or possibly the other way round...But it doesn't. Sequel comes from the Latin sequī, to follow: which makes the whole thing a nonsense.
The word's been around since the eighties, apparently, so I must count myself fortunate to have escaped it until now.