\[\boxed{\mathbf{3.}\mathbf{312}}\]
\[\textbf{а)}\ 3 \cdot 2^{3} < (3 \cdot 2)^{3}\ \]
\[3 \cdot 8 < 6^{3}\]
\[24 < 216.\]
\[\textbf{б)}\ 2^{4} \cdot 2 > 2^{4}\]
\[16 \cdot 2 > 16\]
\[32 > 16.\]
\[\textbf{в)}\ (4 \cdot 3)^{2} = 4^{2} \cdot 3^{2}\]
\[12^{2} = 16 \cdot 9\]
\[144 = 144.\]
\[\textbf{г)}\ 2^{3} \cdot 2^{5} = 2^{8}\ \]
\[8 \cdot 32 = 256\]
\[256 = 256.\]