Three planes plotted together

> with(plots):

> plane1:=plot3d(x-y,x=-1.5..1.5,y=-1.5..1.5,view=-1.5..1.5,color=gray):

> plane2:=plot3d(x+y,x=-1.5..1.5,y=-1.5..1.5,view=-1.5..1.5,color=green):

> plane3:=plot3d(-x-2*y,x=-1.5..1.5,y=-1.5..1.5,view=-1.5..1.5,color=pink):

> display(plane1,plane2,plane3);

>