Even though the cast was largely already established in Itsuwari no Kamen, I think that the characters, at least most of them, still managed to grow meaningfully in 3. Aside from the obvious ones like Haku, Kuon, Anju and Nekone whose situation got changed completely, and the actual new additions, Atui for example ended up changing quite a bit in the game.
Some of what happened was stuff I'd probably have protested originally, like how her view of Haku ended up (and, well, in a way I did), but I liked the way it was done so I can't really complain. Also aside from that she gradually ended up becoming more... human? in the game compared to how she originally was in Utaware 2. Kind of finishing this with the good bye conversations near the end of the game, and during the final fight with the combat lines, is something I really ended up liking. There are some other characters, like Kiuru, who ended up kinda different from their Utaware 2 self, but I'm too lazy to write about him since I didn't like him all that much...

Well this isn't anymore a direct reply, but I also ended up liking the somewhat different way of doing self-sacrifice Utaware 3 did. I'm usually not too big a fan of self-sacrifice, especially if it's overly beautifully or heroically portrayed.
Some of the plot developments in the game were a bit sudden, or maybe even forceful, but in the end I think the overall plot ended up pretty solid, though I do agree that the main charm of the game is definitely the great cast, and I think to a lesser extent the very nice world/setting of Utawarerumono too. Because of how much I liked the cast I didn't really mind the somewhat slow start of the first part like a lot of people did, though I'll admit that I see the point here.

In the end I have a bit hard time thinking about all the games individually, especially 2&3, so maybe I'm being very lenient with some of the factors such as the main character development for a lot of the cast being heavily on 2, but I'll just say that overall the series left me pretty satisfied. The final scene is too much.