Re: How do we avoid the third act curse?
I would be the first to say the version of Dracula in Blade Trinity is awful and barely deserves the name (see also Van Helsing; the two worst ever screen Draculas, and in the same year!).
However, the concept of bringing in Dracula was not a silly idea for Blade; quite the opposite, it was the obvious and indeed only proper climax to the series. Blade the character was invented to fight Dracula (in The Tomb of Dracula). It's perfect that he works his way up in the first two films to finally battle the king of the vampires in the final act. It makes perfect sense and anything else would be disappointing.
The mistake is that it's such a lame attempt at Dracula. Admittedly the classic Lugosi style Dracula of the comic would not work in the urban style of the Blade series. But even so, there are other ways of doing it - Nomak's father in Blade II is a clear copy of Nosferatu, and that worked.
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.