\[\boxed{\mathbf{5}\mathbf{.}}\]
\[3 \cdot 1 = 3\]
\[3 \cdot 2 = 6\]
\[3 \cdot 3 = 9\]
\[3 \cdot 4 = 12\]
\[3 \cdot 5 = 15\]
\[3 \cdot 6 = 18\]
\[3 \cdot 7 = 21\]
\[3 \cdot 8 = 24\]
\[3 \cdot 9 = 27\]