The new characters aren't very dedicated
Do you remember the blood pack from Blade 2? Blade had to work with them, but he was also in command and let them die when he felt they outlived their purpose. But in Blade: Trinity we get two new characters in Hannibal King, played by Ryan Reynolds and Abigail Whistler (Whistler's daughter), played by Jessica Biel, and both of them are terrible. They make up the so-called night stalkers that came from the comics, but they are hellishly uninteresting and take up way too much screen time. In other words, they don't add to the story like the Blood Pack does. Instead, they take away from it.
The Godfather Part III has the same problem. Yes, Michael Corleone and Kay Adams-Corleone (played by Al Pacino and Diane Keaton respectively) are returning, but the rest of the cast is almost entirely new and most of them are pale compared to characters like Fredo, Tom Hagen, or Sonny. And then you have Mary Corleone who was played by Sophia Coppola. Many people say that with her terrible performance, she destroys the entire movie. And while nobody is that bad in Blade: Trinity, nobody is that good in Blade: Trinity either.