\[\boxed{\mathbf{3.}\mathbf{311}}\]
\[\textbf{а)}\ 5^{3} \cdot 2^{3} = 10³\]
\[125 \cdot 8 = 1000\]
\[1000 = 1000.\]
\[Равенство\ верно.\]
\[\textbf{б)}\ 3^{3} \cdot 3^{2} = 3^{6}\]
\[27 \cdot 9 = 81 \cdot 9\]
\[243 \neq 729.\]
\[Равенство\ неверно.\]
\[\textbf{в)}\ 5^{2} \cdot 2^{2} = (5 \cdot 2)^{4}\]
\[25 \cdot 4 = 10^{4}\]
\[100 \neq 10\ 000.\]
\[Равенство\ неверно.\]
\[\textbf{г)}\ 3^{3} \cdot 3^{2} = 3^{5}\]
\[27 \cdot 9 = 81 \cdot 3\]
\[243 = 243.\ \]
\[Равенство\ верно.\]