\[\boxed{\mathbf{3.}}\]
\[2 \cdot 6 = 12\]
\[6 \cdot 2 = 12\]
\[6 + 6 = 12\]
\[3 \cdot 6 = 18\]
\[6 \cdot 3 = 18\]
\[6 + 6 + 6 = 18\]
\[4 \cdot 6 = 24\]
\[6 \cdot 4 = 24\]
\[6 + 6 + 6 + 6 = 24\]
\[5 \cdot 6 = 30\]
\[6 \cdot 5 = 30\]
\[6 + 6 + 6 + 6 + 6 = 30\]