There is another lever. The one on the central tomb platform causes a chest to appear, the other two trigger the platforms so you can get up to the chest.
There are also two malachite veins in there, so you may want to save before approaching or running your crosshairs over them, so you can reload if you don't get at least 1 malachite gem (two are not that hard to get, and 3 is possible if you are patient or just lucky; IIRC someone mentioned getting 4, but I have not seen that).
