\[\boxed{\mathbf{3}\mathbf{.}}\]
\[\textbf{б)}\ 14\ :2 + 21 = 7 + 21 = 28\]
\[40 - 18\ :2 = 40 - 9 = 31\]
\[35 + 12\ :2 = 35 + 6 = 41\]
\[12\ :3 + 26 = 4 + 26 = 30\]
\[50 - 15\ :3 = 50 - 5 = 45\]
\[62 - 18\ :3 = 62 - 6 = 56\]