\[\sqrt{18} \cdot \left( \sqrt{6} - \sqrt{2} \right) - 3\sqrt{12} =\]
\[= \sqrt{18 \cdot 6} - \sqrt{18 \cdot 2} - 3\sqrt{12} =\]
\[= \sqrt{6 \cdot 3 \cdot 6} - \sqrt{36} - 3\sqrt{4 \cdot 3} =\]
\[= 6\sqrt{3} - 6 - 6\sqrt{3} = - 6.\]