I gave you the densities in the other question. What you now need to do is to calculate the volume of a ball that is 3 feet in diameter.
If you're not familiar with it, the formula for this is 4/3*pi*r^3, where r is half the diameter.
4/3 * 3.14 * 1.5^3 is about 3.7 ft^3. With a density of 975 lb/ft^3, that's about 3600 pounds. This assumes the ball is solid.
If it's hollow, we would need to know how thick the shell is.