The first book was fantastic, the second slightly less so, but the third was a massive disappointment. I'm going into spoilers here, so if you haven't read the books you may want to come back when you're finished!
The turning point for me was when Katniss is rescued from her second Hunger Games at the end of the second book. During the first book, The Hunger Games, Katniss is full of agency. She volunteers to take her sister Prim's place in the games, and although she has no choice about taking part in them, or the media circus that accompanies them, she determines to survive if she can and not compromise herself if she can't. She defies the Capitol when she covers Rue in flowers, and when she suggests she and Peeta eat the poisonous berries when cheated of their double victory.
In the second book, Katniss is understandably floored by having to take part in a second Hunger Games. While she has agency, this time her choice is to attempt to keep Peeta alive at the expense of her own life. Again she has no choice but to play, and again she decides to do so on her own terms.
At the end of the second book, Katniss is rescued from the games by what turns out to be the rebellion she accidentally started. That's when she starts to lose her agency.
The rescue itself isn't the problem, although it would have been nice to see the supposedly resourceful victors rescue themselves. The problem is that after the rescue Katniss seems to spend the whole book being told what to do. No one told her about the rebellion, and she had no part in planning the escape. Later, her refusal to fit in with the routines of District 13 comes across more as a child having a tantrum than her choosing rebellion. Although she's allowed to make the choice as to whether or not to be the Mockingjay, and again does so under her own terms by making conditions, she's largely sidelined. She's the face of the rebellion, a media figure.
The sidelining of a child during a rebellion is understandable, although it's arguable that anyone who survives the Hunger Games is a child any more. She does, towards the end, decide to fight the rebellion on her own terms. However her unauthorised mission to assassinate President Snow doesn't end how she intended. Instead she gets to watch her sister be killed, and is rescued again.
This was the point at which I nearly threw the book at the wall. Katniss volunteers to take part in the Hunger Games only to save Prim. That's the reason behind most of her decisions. With that one mostly off-screen death, Collins manages to completely invalidate everything Katniss has done for the last three books. Sure, the Districts have their rebellion and throw off the yoke of the Capitol, but that's not what motivates Katniss. In that one stroke, she loses everything she's fought for.
She spends the rest of the book aimless. She makes one final momentous decision after which she's imprisoned, and not even told she's on trial, and then shipped back to the ruins of her home to live out her life. But she gets to have a boy and later his babies, so that's OK (/sarcasm). It may not be what Collins was going for, but the end of the book screams that rebellious girls can only lose, and are destined only to be wives and mothers.
I don't have a problem with happy endings, and Katniss ending up with one of her suitors will make at least part of the audience happy. But I'd rather have endings that don't wipe out all of the main character's achievements that came before them. I loved the beginning of the trilogy, and while I'll read books 1 and 2 again, I won't be revisiting 3. It's too much of a disappointment.