Stephen King’s 1986 novel It was actually long enough that splitting it into two movies was totally warranted. But according to director Andy Muschietti, King’s book hints at a backstory for Pennywise the Dancing Clown (Bill Skarsgärd) which could potentially be mined for a third film, should the opportunity arise. In a recent interview with io9, Muschietti implies that there’s enough Pennywise-centric material to fill out a possible third film.
Source: /Film