In Luskan, there is this gnome, and he wasntss you to help fix machine. I found the two other levers, whech you get from the ghoul lords, but where is the last one? I went through every single sewer and, I can't find it...

I found one in a trapped chest in a side room of the city's brothel.

Tal is right. It's in the brothel. I also had some trouble finding it.