They should have just left 'The Hangover' as a one-off film - it would have gone down in comedy history that way. Unfortunately, thats not the way Hollywood works and while part 2 had some funny bits (I haven't seen part 3), it wasn't a patch on the fresh and original first film.
I'm glad they never.
The first was a bland average comedy. Never stood out more than American Pie.
The second in the season was good..hence the third and final part.