Act 3 feels unfinished because compared to act 1, the companions are quite silent, especially after finishing their personal quests and getting the lovely romance scene afterwards plus the dialogue with them at the end of their quest. After finishing these things, it's like the companions don't exist.
Act 3 has many great quests and epic fights, so you can't say the storytelling in that regard is lacking. What is lacking in act 3 is satisfying endings to personal stories that feel long-lasting throughout the act.
As an Astarion enjoyer, it feels so empty for him to go from my best friend and lover in act 1 and act 2, to just there after finishing his story. Ascended Astarion does get a bit more dialogue with his romance partner, but spawn Astarion is lacking. The graveyard scene is epic, but after that he is just radio silent, minus the really cool Araj scene. I think this can be made to feel less bad, even for all companions and romances, if there were more camp interactions that felt like the companions are still involved and that the romance has progressed.
