Sorry, I was vague and confusing, here's the task given:
Rigid Structures are constructed using unit rods.
An example is shown below:
* Pic opf a 2 by 2 by 2 cube*
The individual rods in the structures are held together using different types of joints. (Just think of making cubes using matchsticks and playdough*.
A three joint would be on the vertices of each cube no matter what the size and so therefore there'll be 8 three-joints.