\[\boxed{\mathbf{3.}}\]
\[\textbf{а)}\ 35 + 2 = 37\]
\[52 + 4 = 56\]
\[46 + 3 = 49\]
\[21 + 7 = 28\]
\[65 + 2 = 67\]
\[73 + 5 = 78\]
\[\textbf{б)}\ 2 + 33 = 35\]
\[3 + 24 = 27\]
\[4 + 42 = 46\]
\[5 + 62 = 67\]
\[6 + 53 = 59\]
\[7 + 71 = 78\]