Who can explain me how to solve this problem?

Firstly try to solve similiar problem in one dimension. Then you will find 3D-answer easily.
Good luck!

I know how to generalize solution of 1-dimensional problem
to case 3d, but i can't solve 1-dimensional problem :)

I think it's a "not hard combinatorial task" but I cannot get the sample output on test 3, it think it should be 13*13*10

In one dimension is enough to find final positions of left and right ends of segment only. All coordinats between these positions are achievable.