\(\boxed{\mathbf{3.}\mathbf{393}}\)
\[\textbf{а)}\ 17 + 30 = 47\]
\[5 + 25 = 30\]
\[37 + 203 = 240\]
\[384 + 20 = 404\]
\[127 + 23 = 150\]
\[\textbf{б)}\ 728 - 7 = 721\]
\[80 - 25 = 55\]
\[100 - 6 = 94\]
\[70 - 17 = 53\]
\[300 - 85 = 215\ \]
\[\textbf{в)}\ 20 \cdot 5 = 100\]
\[4 \cdot 25 = 100\]
\[50 \cdot 20 = 1000\]
\[24 \cdot 1000 = 24\ 000\]
\[270 \cdot 100 = 27\ 000\]
\[\textbf{г)}\ 26\ :2 = 13\]
\[144\ :9 = 16\]
\[370\ :10 = 37\]
\[600\ :30 = 20\]
\[303\ :3 = 101\]