What is Threequel?

Excuse me, could you please clarify what exactly is meant by the term 'Threequel'? Is it referring to a specific type of sequel in the film industry, perhaps a third installment in a series of movies? Or does it have a broader application, perhaps in the realm of literature or even technology? I'm particularly curious about its origin and usage, as I've seen it mentioned in various contexts but have yet to fully grasp its meaning.