Assuming the can is packed tightly, then the height of the can is simply three times the diameter of one ball.
The volume of the can is its height times the area of the bottom (or top), which is a circle with the same diameter as the ball.
The formula for the area of a circle is πr2, where r is the radius. You can get the radius from the diameter, the area from the radius, and then the volume from the area and the height of the can.
Hope this helps!
Stephen La Rocque.