It depends on where the story falls in relation to the timeline of the rest of the narrative. If the storyline occurs prior to what the other two parts establish, it would be a prequel. Assuming we're going with typical, orderly narrative continuity/progression, the second sequel (or a sequel to the first sequel), albeit having no correct name in particular, may be addressed as anything from "3rd installment" and "Book Three" to "Threequel" or even something as simple as Part III. However, in essence, it is a sequel by nature - just the III'rd release. It goes without saying that three isolated releases - or a three-part series - is called a trilogy.