My observation is that you only get anything on the 3rd level to satisfy a quest from one of the bf merchants. Sometimes all the quest will be spread over several dungeons and sometimes they will all be in one dungeon.
Currently, I'm in Act III and have done all but one dungeon on the bf. I have completed 2 of the quests and they were both in the same dungeon. So I would bet that the remaining quests are all in the final dungeon.
I have not seen anything to suggest that this in not how its suppose to be.
