FactorTermsList[(3 x^4)/(2 Sqrt[Pi]) - (9 x^2 y^2)/(2 Sqrt[Pi]) + (9 y^4)/( 16 Sqrt[Pi]) - (9 x^2 z^2)/(2 Sqrt[Pi]) + (9 y^2 z^2)/(8 Sqrt[Pi]) + (9 z^4)/(16 Sqrt[Pi]),{x,y,z}]