I'm trying to get all the conversation achievements. I'm currently trying to get Leahs.
Anyone know which part of act 3 do you get "Hopes" and "Dreams"?

I did these shortly after game release, so I can't recall all the detail, but here are what my notes says.

Conversations:
Concern (Act III) - Available after accepting The Siege of Bastion's Keep (Part 1).
Hopes (Act III) - Available after accepting Turning the Tide (Part 1).
Dreams (Act III) - Available right after listening to Hopes (Act III).


In short every major quest change changes NPC dialogs and journals, so between doing Skycrown and Stonefort you have to go back to town.

@Bort is right in all three parts.

He's a well organized achievement hunter :)

Thank you. Much appreciated