Brittleness is a property of material which breaks with little or no deformation.
Toughness: When a huge external force is applied on a metal, the metal will experience fracture. Toughness is the ability of a metal to resist fracture. The ability of a material to resist fracture due to high impact loads, is called toughness.
Stiffness: When an external force is applied on a metal, it develops an internal resistance. The internal resistance developed per unit area is called stress. Stiffness is the ability of a metal to resist deformation under stress.