Wow I never thought that there will be quick answers who all report the same feeling (more or less).

So far I read several points that explain for me why act 3 feels bland.

Originally Posted by vx_phoenix_vx
So much of the quests in Act 3 feel like chores and most of them have little to no impact on the story (except the companion quests). The undercity in particular is super frustrating to go through.

This, oh yes this. It absolutely feels like chores. Lifting the shadow curse felt more interesting than beating the brain. Ketheric Thorm was way more interesting as a character compared to Orin and Gortash.