\[\boxed{\mathbf{3.}}\]
\[\textbf{а)}\ 9 + 3 = 12\]
\[8 + 5 = 13\]
\[7 + 4 = 11\]
\[11 - 2 = 9\]
\[12 - 4 = 8\]
\[13 - 5 = 8\]
\[\textbf{б)}\ 13 - 4 + 8 = 9 + 8 = 17\]
\[13 - (4 + 8) = 13 - 12 = 1\]
\[13 + 4 - 8 = 17 - 8 = 9\]
\[8 + (20 - 14) = 8 + 6 = 14\]
\[(6 + 9) - 8 = 15 - 8 = 7\]
\[14 - (11 - 5) = 14 - 6 = 8\]