\[\boxed{\mathbf{3.}\mathbf{313}}\]
\[\textbf{а)}\ 144 = a²\]
\[a = 12.\]
\[\textbf{б)}\ a^{2} = 169\]
\[a = 13.\]
\[\textbf{в)}\ a^{2} = 1\ 000\ 000\]
\[a = 1000.\]
\[\textbf{г)}\ 216 = a^{3}\]
\[a = 6.\ \]
\[\textbf{д)}\ a^{3} = 729\]
\[a = 9.\]