$xyz - x - y - z + 2 \ge 0$
$xyz + 2 \ge x + y + z \ge 3{\left( {xyz} \right)^{1/3}}$
$xyz + 2 - 3{\left( {xyz} \right)^{1/3}} \ge 0$
at $\left( {xyz} \right) = {t^3}$
${t^3} - 3t + 2 \ge 0$
$\left( {t + 2} \right){\left( {t - 1} \right)^2} \ge 0$
$\left[ {t = - 2} \right]{t^3} = - 8$