(x+y+z)^3-x^3-y^3-z^3
\(=\left\{\left[\left(x+y\right)+z\right]-\left(x+y\right)^3-z^3\right\}+3x^2y+3xy^2\)\(=\left[3\left(x+y\right)z\left(x+y+z\right)\right]+3xy\left(x+y\right)\)
\(=3\left(x+y\right)\left(xz+zy+z^2+xy\right)=3\left(x+y\right)\left(y+z\right)\left(z+x\right)\)