First, the Sewer of Horrors is supposed to be the hardest dungeon in the game, even harder than the final area of the main quest. So expect a few screaming headaches.
Second, it takes FIVE, not three, levers to activate the teleporter. I know, the Developer is a jerk. He's an old fashion DM that thinks killing your character. Don't worry the forum is here to help.
If you want to use out-of-game knowledge, the Wiki link to the area is
here.
If you want to problem solve using in-game knowledge, here's a few hints.
1) By the time you've reached the Sewer of Horrors, you've likely discovered one or two other places where levers were hidden behind secret doors.
2) You've might have found out how to search for missing secret doors using a Scroll or the Cleric's find secrets skill. The text telling if there's any remaining secrets is quite useful.
3) Secret levers are best hidden in places you're likely not look. Either out-of-the way, or near insane monsters that you just want to run past.
I moved this from a tag-on to a two year old thread to it's own thread for organization.