A bit late, but
First lever can be found in the first room on the right
For the second lever, go to the left room and click the middle boulder till you see a lever
For the third lever, take the skeleton arm on the ground and use that to push the block on the wall in the first room on the right
Then, once you use them on the cogs in the far right, the slab will open in the far left room. Inside should be the code.