Graphics3D[{Gray,Opacity[0.2],Cuboid[{0,0,0}],Red,Opacity[0.4],Polygon[{{0,0,0},{1,0,0},{0,1,0}}]},Axes->False,Boxed->False]Graphics3D[{Gray,Opacity[0.2],Cuboid[{0,0,0}],Red,Opacity[0.4],Polygon[{{0,0,0},{1,1,1},{1,0,0}}]},Axes->False,Boxed->False]Graphics3D[{Gray,Opacity[0.2],Cuboid[{0,0,0}],Red,Opacity[0.4],Polygon[{{0,0,0},{1,0,1},{0,1,1}}]},Axes->False,Boxed->False]