A cube of side length 6 cm is painted red on all its sides. This cube is cut into smaller cubes of side length 2 cm. On each of the smaller cubes, the unpainted faces are painted green. Each of the smaller cubes is now further cut into cubes of side length 1 cm. How many small cubes of side length 1 cm will have exactly three of their faces coloured green? (A) 48 (B) 64 (C) 56 (D) 36