Radius of cylinder = Radius of ball = r.Height of cylinder (h) = 3 × Diameter of ball = 3 × (2r) = 6rVolume of cylinder = $\pi r^2 h$$= \pi \times r^2 \times 6r$$= 6 \pi r^3$Volume of 3 balls = $3 \times \; \frac{4}{3} \pi r^3 = 4 \pi r^3$Volume of air = Volume of cylinder - Volume of 3 balls$= 6 \pi r^3 - 4 \pi r^3$$= 2 \pi r^3$