No, I haven't seen Shark Attack or Shark Attack 2, but from what I can tell, neither of those movies has anything to do with Shark Attack 3: Megalodon, aside from featuring sharks attacking (and the original Shark Attack apparently doesn't even feature much of that). And while Shark Attack and Shark Attack 2 are undoubtedly terrible, Megalodon is the one that has built up a reputation as one of the great bad movies of all time, thanks in large part to one particularly infamous line that star John Barrowman (who's gone on to be very successful on Dr. Who, Torchwood and the British stage) will apparently never live down.
But there's more to Megalodon than just Barrowman's Ben Carpenter delivering the world's most awkward come-on. For starters, the movie takes place in Mexico but was shot in Bulgaria, and almost every supporting character is played by a local actor whose dialogue was post-dubbed, giving the movie a very stilted, unnatural feel even with the rare bits of dialogue that aren't horribly written. The "Mexican" resort where the movie takes place is decorated with Mexican flags and pictures of Mexico's then-President Vicente Fox. This seems to be the movie's primary mode of illustrating character backgrounds and roles; the former Navy man who helps the protagonists defeat the megalodon has giant photos of George W. Bush and Dick Cheney and a huge U.S. Navy logo on his wall to indicate his patriotism. Even more amusingly, paleontologist Cat Stone (Jenny McShane, who had a completely unrelated role in the original Shark Attack) is introduced with what looks like a picture book about dinosaurs sitting on her desk. That level of knowledge about anything scientific is pretty much standard for this movie, though.
Barrowman plays a resort employee who stumbles on the title creature, a prehistoric giant shark thought to be extinct. He uploads a picture of the shark's tooth to a hilariously rudimentary website, which attracts the attention of the San Diego-based Cat, who shows up hoping to study the creature. Of course, it's far too dangerous to study, and various victims soon succumb to megalodon attacks, while the fat cats who own the resort refuse to shut down the beach. The movie really kicks into hilariously awful mode when Ben and Cat discover that the megalodon they were chasing was just a baby, and the actual giant shark ("the size of a Greyhound bus," Ben says) shows up. Most of the shark scenes are just recycled stock footage, and the supposedly monster-size shark is just the same footage shown in close-up, with various items (boat, inflatable raft, jet ski) superimposed over it and made to look tiny in comparison (the shark swallows all of those things whole).
Barrowman insists that the infamous line was a joke not meant to end up in the final film, but it's hard to tell just how much of the rest of the movie is intended to be funny. It's not quite goofy enough to be a full-on satire, but certainly bits like the guy directly jet-skiing into the horribly green-screened shark's mouth aren't expected to be taken seriously. Given the proliferation of terrible shark-attack B-movies, it takes something special to rise above the pack and into the realms of the great bad movies of all time; whatever that is, Megalodon clearly has it.