I hope that there is enough information in this answer:
Sphere Vol = 4/3Pi radius^3
Give the 4/3Pi bit a constant - 'C'
Vol = C x radius^3
V1 = C 3^3 = 27C
V2 = C 5^3 = 125C
New Volume = 27C + 125C = 152C
Aproximate radius = cube Root 152 = Approx 5.3
(5 cubed = 125, 6 cubed = 216)