Not knowing where you got the 2 tablets you have, I'll have to guess. The third tablet piece is found in a hidden room which you open by opening with glowstones two rooms that are near to one another and switching the levers inside.
Not sure where you mean by the crab cage. There was a room filled with cages that levers on the wall raised. One such cage had an opening in the bottom which allowed you to swim into other areas.
Wyv
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