\[f(x) = \sqrt{x - 3} - \sqrt{3 - x}\]
\[\left\{ \begin{matrix} x - 3 \geq 0 \\ 3 - x \geq 0 \\ \end{matrix}\text{\ \ \ \ \ \ \ \ } \right.\ \left\{ \begin{matrix} x \geq 3 \\ x \leq 3 \\ \end{matrix};\ \ \ \ \ \ \ x = 3 \right.\ \]
\[\Longrightarrow \ \ \ \ \ \ f(3) = 0.\]