Personally, I really liked IM3. I thought the Mandarin twist was brilliant, Tony's character arc went REALLY deep and was well-done, and I liked how dark the tone was overall.
...if I had to pick a single movie out of the entire original 23 films that was completely unnecessary to the overall story arc of The Avengers Initiative...this would be the one.
For my mileage, I completely disagree because:
Tony's PTSD is honestly very important to HIS character development. His relationship with the kid is brilliant and pays off at his funeral.
This. I think it was HUGE to show him being mentally fragile as a result of the battle of New York. And those "cracks" in his psyche informed and influenced pretty much every decision he made throughout the Infinity Saga, from AOU, to Civil War, to Homecoming, to Infinity War, to Endgame. The Tony Stark character was done SO well, and IM3 is a huge reason why.
Mine was how badly they wrote the Killian character at the end. I really loved the idea they started out with him, but then when he became a human dragon thing and he was fighting 40 Iron Man suits and everything was exploding, I just stopped caring. If they had gone for a smaller, more personal ending, that would've made it great. The rest of the movie was fantastic.
Yeah, I mostly agree. The back story and all his motivations up to that point were great. And when it was initially revealed that he was the puppetmaster behind the whole thing, that was good as well. But he just started to feel cartoonish and, as much as I hate to use this term, "generic" in much of the final battle, and that left me caring less.
I still like this movie a lot, and feel that the quality is definitely there. But the last few minutes (not even the entire third act) just leaves me losing a bit of my emotional investment for the reasons you mention. Overall, not a huge knock on the film though. All that just makes it middle-of-the-pack MCU, which is still very entertaining.