Failing to outdo its predecessor in any real way is one thing, but Terrifier 3's fatal flaw is this: it's an incredibly boring slog.
The film is largely split in two: Sienna not doing a whole lot, and Art running around being a ratbag. It begins with some of the latter, then Sienna leaves some kind of care facility and lives with her aunt, uncle, and niece, Gabbie.
Gabbie is unbearable almost immediately: she's just too saccharine. The self-assured cheeriness,…