(x+y+z)^3-x^3-y^3-z^3=3yx^2+3xy^2+3xz^2+3yz^2+3zx^2+3zy^2+6xyz=3xy(x+y)+3z^2(x+y)+3z(x^2+y^2+2xy)=3xy(x+y)+3z^2(x+y)+3z(x+y)^2=3(x+y)[xy+z^2+z(x+y)]=3(x+y)[x(y+z)+z(y+z)]=3(x+y)(x+z)(y+z)