\[\boxed{\mathbf{3.}\mathbf{275}}\]
\[\textbf{а)}\ 100 - 1 = 99\ :3 =\]
\[= 33 + 27 = 60\ :4 =\]
\[= 15 + 135 = 150\ :25 =\]
\[= 6 + 14 = 20 \cdot 5 = 100\]
\[\textbf{б)}\ 90 - 45 = 45\ :15 = 3 + 13 =\]
\[= 16 \cdot 5 = 80\ :20 =\]
\[= 4 + 26 = 30 \cdot 3 = 90\ :1 = 90\]