The Hard difficulty is not available until you have completed the Normal difficulty.
The first completion of a Season III stage, on both difficulties, awards the player with Valhalla Coins. Please see our article What are Valhalla Coins? to find out more about these!
For further gameplay help and discussion regarding Season III, we highly recommend visiting our Empires & Puzzles Community Forum.
Please note - To unlock the Hard difficulty of a selected stage, you must complete all of the previous stages in Hard difficulty and the current stage on Normal difficulty.