You will need about 6 cubic feet of stones or pebbles to fill the hole (3ft Length x 8ft Width x 3 Inches depth).
We used the formula Length (Feet) x Width (Feet) x Height (Feet) = Cubic feet.
Length = 3 feet
Width = 8 feet
Height = 0.25 feet (3 inches)
When you multiply all these above three units measurement, you get:
3 ft x 8 ft x 0.25 ft = 6 cubic feet of stones.