This activity has been particularly created for the highest attaining. (The pupils that you come across in many classrooms just once every few years.)
This activity also follow on for those who have successfully worked at Doplication.
Let me help you visualise this representation of a 3D situation.
It's kind of two set of three adjacent cubes with balls at each vertex.
Towards the bottom left of the picture are the eight large balls, then going backwards there are eight medium red balls and further back are eight small ones.
In the centre of the front three cubes are three orange balls and at the centre of the rear cubes are three brown ones.