Hi Domenico,
It is a 3 dimensional array where the first index is 0 (rather than 1). You can access it with at:at:at:, for example
a3dArray at: x at: y at: z
and set it similarly, for example
a3dArray at: x at: y at: z put: 1
In the CA tool, there should be some additional examples of how to use it.