Hello!
Used PolyhedralSets for plotting.
At the moment there are a number of questions in the construction of graphs:
How to adjust the rotation of this figure around the z axis?
How to make, what color of a figure received in a cut would be much darker or Darker color?

with(PolyhedralSets);
P1 := PolyhedralSet({-x-20 <= 0, -x-20 <= 20-y, -x-20 <= -(1/2)*y, -x-20 <= -z, -x-20 <= -(x+y)*(1/2), -x-20 <= -(y+z)*(1/2)});
Plot(P1, axes = normal, view = [-40 .. 40, -40 .. 40, -40 .. 40], orientation = [30, -30, -60], color = blue, thickness = 2);
Plot(P1, axes = normal, view = [-40 .. 40, -40 .. 40, -40 .. -40], orientation = [0, 0, 0], color = blue, thickness = 2)

I would like to look at a 3d plot including an condition about the tree variables of the plot

For this simple case, I only want to see the plot with the condition that 
{40 < y, x < y-40, z < y-40} or {x = 0, y = 40, z <= 0}, {y = x+40, 0 < x, z < x}, {y = x+40, z = x, 0 < x}

. Is that possible? 

In 2d there is no problem. And in 3d I do not know how.