Please no TF mentioning.

For me:

Matrix trilogy
Alien (will not acknowledge the horrible 4th)
Spiderman
Xmen
Jurassic park
Pirates of the Caribbean

 

Hustler Video's Cookies and MILF trilogy.



Nah. I'd have to say Back to the Future. That's right, I hated Part III.

 

Two immediately jumped to mind: Jurassic Park and The Matrix.

 

I actually like the Matrix sequels for some reason, maybe I just like christian symbolism.

I think a case of this was Blade, the third isn't that bad, but compared to Blade I and II, it almost hurts.

Transformers narrowly adverted this by having a pretty good third movie, but ROTF's suckyness will be talked for ages.

The mummy franchise sufered a mild case of this on the third, the 30's action flick formula just didn't fly there, they were always fun flicks, but the third...sort of isn't.

Underworld is worth of notice: The first movie was so-so but opened an interesting universe with interesting mithology, the second was a trainwreck, the third was a solid movie but still missing something, the fourth was finally actualy pretty good, it's a franchise that started off meh and ended up pretty cool.

 

Pirates of the Caribbean

 

Oddly enough, I also prefer the POTC sequels to the first.

 

I guess Alvin and the Chimpmunks. The first is a decent matninee but the second? Ugh. Haven't seen the third one so I really don't care.

 

Star Wars OT. Return of the Jedi is practically a self-contained example of it, counting the acts.

I'd have to say I'm not surprised at all.

 

Fockers.

 

Batman and Superman.

 

Almost any superhero movies, the third is the last and the most controlled by the studio. If there is a 4th it's usually awful.

 

The Matrix, Jurassic Park, and most superhero films come to mind (Spider-Man and Batman most especially).

Or, exactly what everyone else has already said  

 

Transformers.
Batman (89' quadrilogy)

 

those films have or are getting a fourth film (fifth in pirate case) so they are not a trilogy anymore, thus they dont count

 

Mine would be the Mummy trilogy. The third wasn't bad but it sure wasn't as memrable as the first two.

 

X-men is the first thing that comes to mind. It started off good then it got even better with X2 and then it fell apart with the third movie.

Alien 3 is awful even for a third movie in a franchise.

Terminator 3 was also bad.

I loved the first Matrix movie and I liked Reloaded, but I didn't like anything in Revolutions besides the battle scenes.

 

Didn't you read the OP's post?

Anywho:

Pirates of the Carribean
Matrix

 

Do the 4 Batman movies with the same Alfred every time count? I mean, you can always remove one and the in the end result, the last movie will most likely suck.

 

The Matrix trilogy. The first movie was great (my first DVD) but the others are a let down in comparison.

 

I'm usually more forgiving that most when it comes to trilogies, so while I won't say awful (with two exceptions), these are the trilogies that started off awesome, but ended in mediocrity.

X-Men
Blade
Spider-Man (Here's that exception. Just awful)
Godfather
The Mummy
Batman
Ocean's 11 (admittedly, 13 was loads better than the abysmal 12, but still nowhere near as good as the first movie)
Teenage Mutant Ninja Turtles (the other exception. *facepalm*)
Mass Effect (I know, it's not a movie, but still.   )

That's all I can think of, since I like/love the Matrix, Pirates (not counting On Stranger Tides), Transformers, and Star Wars trilogies (Both the OT and PT, yeah, I know).