Something about form

[neopatogen]

I can't understand how to build cast shadows in 3-point perspective...I mean, a precise geometrical way of building such shadows, just to understand how it works. Got stuck while analyzing the first picture (cube). My research gave no result, all resources seem to focus on shadows in 1- and 2- point perspective. Could anybody help?...

Anyway, I'm going to learn more perspective, as much as I manage until this week's deadline.