\[\boxed{\mathbf{3.}}\]
\[\textbf{а)}\ |x| < 5\]
\[\left\{ \begin{matrix} x < 5\ \ \ \\ x > - 5 \\ \end{matrix} \right.\ .\]
\[\textbf{б)}\ |x| \leq 4\]
\[\left\{ \begin{matrix} x \leq 4\ \ \ \\ x \geq - 4 \\ \end{matrix} \right.\ .\]