> N[3]:=(xi,eta) -> -(1+xi)*(1+eta)*(1-xi-eta)/4;

     N[3] := (xi, eta) -> -1/4 (1 + xi) (1 + eta) (1 - xi - eta)

> plot3d(N[3],-1..1,-1..1,grid=[15,15],axes=boxed,scaling=constrained,
> style=patch, orientation=[-60,60]);

> 
> 
