As for me, I like all three because I'm an Iron Man fan. I read a lot of the comics when I was young and enjoy the films now, which I'm sure RDJ has a lot to do with it but then again, I think the casting as a whole has been very good. But I do feel that IM3 was the weakest of the bunch in part because it didn't really go in a direction that I expected or would have preferred based on the previous two.
For starters, let's just face it, two good IM movies capped off by The Avengers. Kind of hard to top that. Where do you go with it all? Or more specifically, where do you go with it all AND make it pleasing to everyone? After that, IMO, RDJ (or the scripting) just wasn't as funny in IM3. I fully understand that storyline took on a bit of a different dynamic (attacking his home, Pepper and PTSD from New York) but it just wasn't quite Tony as I would expect him to be. And as I type I realize that I'm not saying funny as in comedic but rather funny because he's so arrogant and narcissistic that it comes across as funny. It works for the character (and actor for that matter) and it was just missing in large degree.
I also felt like the focus of the armor was not right. The first movie, it was new, fresh, exciting and bada$$! Then in IM2 it was modified and better, IMO, but limiting my few issues with IM2 to my topic, the worst part of that movie to me was the party scene when he was drunk wearing the armor. Not to say I don't get it and it didn't fit, fortunately the scene was short lived and tied into an action element at the end but I use that scene to set up part of my issue with IM3. And that's his casualness at time about the armor. Pepper comes home and it's sitting on the couch with it's legs crossed? After dragging it into the garage in Tennessee, he props if up like it's a person. There are a few more like this but these types of scenes turn the armor into a gimmick to me. It almost makes the armor no longer "cool", IMO. The Avengers you got a peek at it's underwater abilities. Cool. How he took it off walking along the platform. Very cool. Sit it on the couch and cross its legs? Now it's a step up from the robotic arm "Dummy". Maybe that's just me.
When I went to see IM3 at the theater I also did not like all the suits towards the end. I also didn't care for how Pepper's character was handled towards the end either. After viewing it at home, I seemed to not mind all the suits as much; although, I still don't care for how "easily" he can jump off a scaffold and get in one, but I better understood why all those suits existed. I still don't care for how Pepper's character was used at the end and how it was all cleaned up nice and neat at the end. I get it, you need that I guess but I just expected something stronger or more creative. But I'm no writer, director or producer so take that with a grain of salt.
One other thing I've been mulling over since seeing IM3 at the theater is if the superhero craze is waning. After considering Man of Steel and Wolverine (which I haven't seen, only heard and read about), it just makes me wonder if it's getting a little stale. As I've said before, I believe Thor is going to be a big tell for whether this is the case or not. Did we just get a "rough" stretch of movies? Or is it all changing?