\[\boxed{\mathbf{3.}\mathbf{354}}\]
\[\textbf{а)}\ 5 + 18 = 23\]
\[123 + 7 = 130\]
\[66 + 34 = 100\]
\[78 + 97 = 175\]
\[\textbf{б)}\ 276 - 13 = 263\]
\[80 - 25 = 55\]
\[630 - 60 = 570\]
\[123 - 34 = 89\]
\[\textbf{в)}\ 12 \cdot 4 = 48\]
\[70 \cdot 10 = 700\]
\[25 \cdot 4 = 100\]
\[14 \cdot 3 = 42\]
\[\textbf{г)}\ 60\ :10 = 6\]
\[210\ :7 = 30\]
\[204\ :2 = 102\]
\[150\ :25 = 6\]