This is a volume problem using a truncated cone .
If you invert the flower pot upside down, you get a truncated cone:
The solution to the problem is to find the volume of the outer cone then subtract the volume of the inner cone.
The difference between the two cones is the 3 inch thick uniform wall.
There is a formula for the volume of a truncated cone:
Volume of outer truncated cone:
Volume of inner truncated cone (subtract the 3" wall from each dimension):
Volume of material (one pot) :
Finally, Volume of 1000 pots :
Hope that helped