Wow. 
What did I think of book three? I'm still trying to work that out. 
It was better than Insurgent in a lot of ways, not as fast paced as Divergent until the very end, but it did offer a lot of more of the background of the world they live in. This also helped to explain why the things I found so hard to believe about their society in the first two books were as they were. 
As with Divergent, if you don't logic at it too much, and go with the story, it's an enjoyable read. I could pull it apart and point out all the flaws, but I'm not going to. I am too caught up in the aftermath of this book, and... You know what? 
You succeeded, Veronica Roth, you made me cry. 
It wasn't Tris dying that made me cry, that gave me the feels, but rather Four's reaction to it. 
I'm still not sure how I feel about it. Part of me is annoyed that she sent herself in there instead of Caleb, part of me knows it's only fitting for her to do such a thing. Part of me is glad that the author didn't take the easy way out so she could keep her MC alive and give everyone a HEA.
It's not a perfect book, nowhere near, but it kept me interested, it made me cry, and the final 150 or so pages made me think that I may consider re-reading this trilogy at some point in the future, whereas before that I was almost sure I never would. 
Part of me is glad it's over, so I can get out of the mind of Tris, someone who bothered me a lot through the second book, and not so much in this one, but for whom latent feelings of annoyance lingered. 
I think that might be why I had a bit of a hard time getting into this book. It could also be the fact that I was working in retail over Christmas, and when I got home at the end of the day I didn't feel much like reading. 
I'm giving it a four because I think the ending redeemed the series somewhat, and I did enjoy the books for the most part. Also because it is, without a doubt, better than the second book.
I'm glad to have read it, I'm glad to be walking away from it - mostly resolved, and  now I need to go get some distance from it and work out the feels. 
ETA - Having thought about it a bit these last few days, I am downgrading this review by one star, and it has been moved to the "I wish I'd stopped at book 1" shelf. I realised yesterday that this is exactly how I feel. 
Book one was fun and fast paced, book two was certainly not. Book three had a slightly redeeming ending, but which seems more pointless as I go on, and which really wasn't enough to redeem the books that much. 
If you've read book two, read this one. But... In future, I think if I do re-read, it will be book one only. Sorry Veronica, it had so much potential.