(Holy Crap, this is Post #1000! Thanks for putting up with this malarkey for so long, folks.)
So here's the thing about Superman 3: it's not as bad as you think you remember. It doesn't hold a candle to the first two, but for its misfires - and there are plenty - there are enough good moments that make it better than its reputation would have you believe.
Most folks' biggest complaint about the film is that director Richard Lester and writers David and Leslie Newman place too much emphasis on big, campy gags that waste valuable screen time and stall the story. And yeah, that's pretty accurate. I have no problem with writers injecting humorous moments into movies like this, but a lot of the gags here are terribly forced, awkwardly derailing the pace of the movie. Worse still, they often don't even lead up to a satisfying pay-off.
Take the scene during the credits sequence: a small accident begets another, slightly larger one, and so on down the line, building toward a Rube Goldberg-esque (or at least Tom & Jerry-ish) sequence of happenings that you think is going to lead to A Job For Superman. And then... Superman rescues a guy from his car. Sure, it's great Superman could rescue that guy, but honestly, a firefighter, police officer, or even random passer-by with something that could break a window could've done the job. And some other guy gets a pie in the face. The end. Promising set-up, lackluster finish.
(Not that every joke falls flat. The running gag where Lana and Clark always have two conversations running at once with Clark inevitably getting confused was cute, thanks in large part to having two charming actors like Annette O'Toole and Christopher Reeve to sell it.)
Where Superman 3 soars (sorry), though, is the fact that it contains some of the most recognizably Supermanly moments of the entire series. The first of these comes fairly early, as Superman battles a chemical plant fire. The firefighters' water tanks burst, and the whole place is about to erupt in a giant superheated chemical death cloud. So what does Superman do? He finds a nearby lake, freezes it with his superbreath, flies it to the plant and then drops it over the blaze, the heat melting the ice into rain before it can hit the ground. That's about as Superman as Superman gets right there.
And of course, there's the movie's bread and butter, the "Superman goes bad" sequence. While prompted by the villains' artificial Kryptonite (not one scientist in Robert Vaughn's employ thought to question the whole tar thing?), it's a classic Red Kryptonite story brought to life, from Superman going bad right on up to the (literal) split personality showdown between Superman and Clark in the junkyard. It seems like it should be cheesy, but again, the strength of Reeve's performance saves this, too. The darker Superman isn't cartoonishly, cacklingly evil, he's just plain mean, which is more entertaining and sinister at the same time. And as the Clark half of the battle, Reeve isn't just the good Superman in Clark clothing, he's actually retaining the Clark persona, awkwardly standing up to a bully (and outright losing his shit when it becomes clear that's the only way to deal with the situation. Mad Clark is mad.).
The climactic battle with the super computer isn't all that bad, either. A little hokey, what with Robert Vaughn fighting Superman like he was in a video game (complete with sound effects from the Atari 2600 version of Pac-Man), but it does the job well enough as a big end fight scene. You've got Superman fighting off missiles, the giant computer that can figure out any opponent's weakness, a little actual Kryptonite, and the villains' plan going beyond their control. Nothing new, but adequate. Apparently producer Ilya Salkind originally wanted Brainiac to be the villain for this movie, and at least a little of that seems to carry over into the finale, as the computer becomes self-aware and fights to preserve its own existence. So there's that.
Overall, Superman 3 has its problems - and honestly, very few of them have anything to do with Richard Pryor, who does the best with what they gave him (which wasn't always much) - but there's enough goodness in there, especially if you like a little Silver Age in your Superman, to justify giving it another shot. It's really not as bad as people say, and it's a lot easier to take than Superman 4: The Quest for Peace, or even - no, or especially - Superman Returns. If Superman Returns had a face, I would punch it. I would punch Superman Returns in the face. Superman 3, on the other hand, I'd buy a beer. Probably because Mean Superman would make me, but still.