There are 9 levers behind 9 secret doors.
Starting in the nw corner and going east, the lever values are 1,2, and 3.
Next row is 4,5, and 6.
Bottom row is 7,8, and, 9.
You need to pull the right combination of levers to ON and no others.
You need 3 specific levers that add up to 10.
Two are odd and one is even. The even number is bigger than the sum of the odd numbers.