Insufficient resources for full resupply
You just supply with what you can. In your case, supply just 2 coal, instead of 3, because one player has been hoarding coal.

From the rulebook, page 6:


  If there are not enough resources left in the supply, that resource is
  not fully re-supplied! This might happen, if the players store too
  many resources in their power plants. The resource tokens in the game
  are limited.