In House of 1000 Corpses (2003), Spaulding's teeth were starting to rot and were badly stained. In The Devil's Rejects (2005), they were even worse, now chipped and blackened. In 3 From Hell, Spaulding's teeth are straight and gleaming white after 10 years in prison after the events in The Devil's Rejects.
During the final showdown, the assault rifles being used were not available during the time-period this movie was set.
(at around 1h 5 mins) The movie takes place between October 2, 1988 and early November, yet the newspaper that is shown when they're on their way to Mexico clearly says February on it.