What does $\{(x_1, x_2, x_3) \in\mathbb R^3: x_3 \leq x_2 \leq x_1 \}$ look like? What does $\{(x_1, x_2, x_3) \in\mathbb R^3: x_3 \leq x_2 \leq x_1 \}$ look like? 
It seems to be a linear convex cone with vertex at the origin.
I am trying to visualize it but cannot. Thanks!

RegionPlot3D[x <= y && y <= z, {x, -2, 2}, {y, -2, 2}, {z, -2, 2}] seems to do the job in Mathematica.
