So i'd say the Iron Man trilogy is on par with most other superhero trilogies.
It had one great film in Iron Man (Spider-Man 2, X2, The Dark Knight, Blade)
One bad film in Iron Man 2 (Spider-Man 3, X-Men 3, TDKR, Blade Trinity)
And one good film in Iron Man 3 (Spider-Man, X-Men, Batman Begins, Blade 2)
While most others start of good, achieve the best on the second outing and falter at the end, Iron Man is the first to start off the best, falter on the second outing and improve with the last.
I agree with this assessment. Mind you I can still enjoy some of these 'bad' films, I just agree that they are the worst of their respective series.