Re: How do we avoid the third act curse?
I think it's down to three things.
1. The story has already been told, or atleast the most important part.
2. The third movie is often unavoidably repetative of past glories.
3. Because of point 2, the filmakers have to throw in new things to keep the audience intrested, and often the new stuff takes the focus off the main character(s). X-Men III, Spider-Man 3, Blade Trinity, Superman III - all have simply too many characters. X3 especially has quite a ridiculous ammount of character juggling going on. Superman III focuses on the wrong characters and the wrong parts of the story - the romance with Lana and Clark and the battle between Supes and Clark should be the major elements.
I'd argue that Batman Forever actually brings all it's characters and story elements together better than any other superhero movie. It's quite brilliant how everything builds up to the climax, which encapsulates everything in the story and all five main characters, and the theme of duality. So could the first Spider-Man movie burrowed it, but to lesser effect.
There is no Marvel/DC rivalry. Most of the great comic creators worked for both; Jack Kirby, Frank Miller, Grant Morrison, Walt Simonsen, Gil Kane, Steve Ditko, John Byrne, Steve Englehart, Mark Waid...even Stan Lee.