Radius of Lead sphere (R) = \frac{12}{2} = 6 cm

Volume of Lead Sphere (V) = \frac{4}{3} × π × (6)³

Let the diameters of the three small spheres be 3x, 4x, 5x.

⇒ Radius of small spheres = 3x, 4x, 5x

Total volume of spheres = V₁ + V₂ + V₃

⇒ \frac{4}{3} × π × (6)³ = \frac{4}{3} × π × (3x)³ + \frac{4}{3} × π × (4x)³ + \frac{4}{3} × π × (5x)³

⇒ \frac{4}{3} × π × (6)³ = \frac{4}{3} × π × (27x³ + 64x³ + 125x³)

⇒ 216 = 216x³

⇒ x = 1

∴ Radius of small spheres = 3 cm, 4 cm and 5 cm