You need to kill all three victims as you play through Amnesia: Justine.
Prisoner #1 - Kill this guy by pulling the lever a few times. You'll know he's dead when the ladder drops down and lets you move to the next area.
Prisoner #2 - This guy can also be killed by pulling the lever. Slides aren't necessary for this. Just keep pulling the lever until he's dead and the bookshelf in the room across the hall opens up.
Prisoner #3 - Simply keep running straight towards the exit without closing the gate on the right during the water chase/escape sequence.