***Grade B “Jaws” rip-off is mildly amusing, but overall meh*** Evidence of a prehistoric shark is discovered near a resort town on the west coast of Mexican. A beach patrol chief (John Barrowman) and a paleontologist (Jenny McShane) team-up to see if the megalodon actually exists. It does and it’s feeding on beachgoers. “Shark Attack 3: Megalodon” (2002) is not as bad as armchair critics say or as amusing as its supporters say in a “so bad it’s good” way. It’s somewhere in between. Jenny McShane returns from the first flick (1999), but the film’s not as good. Not that the first one was…