Once all 3 keys have been used and have disappeared from your inventory, the wall behind you should slide open and reveal a room with several stat barrels, a chest, and a door. The door is the exit from the area.

Why this sequence does not happen in your game (if the keys are gone from your inventory after use) is a nystery to me and I just don't know why it does not happen as it should. I have never had any problem with those keys in all the times I have played MM7 (which is plenty of times )