Absolutely Spiderman 3. I mean, there have been worse third movies, but I think not a single other one managed to jump from amazing to shitfest with such finesse.

It would definitely be Spy Kids 3 and Narnia3.
______________________________
Portable Ice Maker

Karate Kid

Ugh. Yes.

Even back when I thought Home Alone 3 was awesome, I knew 4 was absolute garbage.

Aliens 3 was like WTF is this? :(

Can't say Matrix Revolutions only because I thought both sequels were equally rubbish compared to the original.

Godfather Part 3 was a huge letdown because the first two were so damn good.

Shrek the 3rd - didn't laugh once.

Neither did I, 4th movie wasn't any better either, Sherk 2 was that funny.

Starship Troopers 3 anyone?

The Dark Knight - Hardy playing Bane as a cross between Dr. Evil and a steroid driven Kenneth Williams among other crapola.
The prior film didnt set ANY kind of good standards though.
I agree with that being utter gash - the only good thing about that was the basic portrayal of sandman being close enough to the comics - but the plot etc let even that down!

-Shrek the Third
-Spy Kids 3D
-X-Men: The Last Stand
-Spider Man 3

I was about to say Pirates 3...but 4 was so much worse (and there are probably more sequels on the way).

dunno about spy kids- having seen none of those films, but i can agree with the rest!

I thought I was the only one who'd ever heard of, much less watched, much less watched several times the original Zardoz.

Maybe I am a weirdo but I thought the third Indiana Jones was a LOT worse than one and two. It was more of a comedy and basically repeated the exact same formula from the original. The fourth movie was even worse though, so what do I know.

Beverly Hills Cop 3 might be literally the worst third every made though, for a big budget popular series.