Three-layered sugar chests are single-tile stationary blockers that take three matches involving sugar keys to open and therefore destroy. Each matched sugar key results in a degrade. After the first degrade, it becomes a two-layered sugar chest with a crumbly biscuit topping, then after the second degrade, becomes a plain sugar chest with no toppings. These blockers cannot be destroyed by anything other than matches involving the sugar key.
Here is the difficulty of destroying a three-layered sugar chest: