You can do the levers in any order, so I'm not sure which one you're missing. But here are some hints:
- One is found by the strange-looking tree. Try bringing another strange-looking item to it.
- Another is found inside the manor. You should eventually come across a note that will help you unlock a hidden room.
- Another is found outside the manor grounds entirely. You'll first need to find a way outside the fence. Make sure you bring a snack!
Hope that helps!