\[\frac{\sqrt{3} - 3}{\sqrt{5} - \sqrt{15}} = \frac{\sqrt{3}\left( 1 - \sqrt{3} \right)}{\sqrt{5}\left( 1 - \sqrt{3} \right)} =\]
\[= \frac{\sqrt{3} \cdot \sqrt{5}}{\sqrt{5} \cdot \sqrt{5}} = \frac{\sqrt{15}}{5}.\]