Most film series suffer in the 3 instalment onwarRAB, personal notes:
Back to the Future, 1 excellent, 2 pretty good, 3 poor
The Matrix, 1 excellent, 2 pretty good, 3 ok
Star Wars, 1 excellent, 2 excellent, 3 ok, prequels ignored
Jaws, 1 excellent, 2 good, 3 crap
Underworld, 1 excellent, 2 excellent, 3 poor
Blade, 1 excellent, 2 excellent, 3 poor
Pirates of the Caribbean, 1 excellent, 2 good, 3 ok
Indiana Jones, 1 excellent, 2 pretty good, 3 ok, 4 poor
Final Destination, 1 very good, 2 excellent, 3 ok, 4 good
Alien, 1 excellent, 2 excellent, 3 good, 4 poor
Predator, 1 excellent, 2 good, 3 good
Aliens vs Predator, 1 poor, 2 poor
Resident Evil, 1 ok, 2 ok, 3 poor, 4 poor
Friday the 13th, 1 good, 2 ok, 3 crap, 4 poor, 5 poor, 6 poor, 7 poor, 8 crap, 9 poor, 10 good
Same goes for Nightmare on Elm Street, Halloween, Wrong Turn, Hostel, SAW, Texas Chainsaw Massacre, Jurassic Park, The Dead Series (Night, Dawn, Day, Land, Survival, etc), Return of the Living Dead Series, Shrek, Robocop, Terminator, Die Hard.
There are some that buck the trend though, The Lord of the Rings and Toy Story for example.