3. (5 points) In the table, 8 is located at the 3rd row and 2nd column, 2017 is located at the $a$-th row and $b$-th column, then $a-b=$ $\qquad$
\begin{tabular}{|c|c|c|}
\hline 1 & 4 & 5 \\
\hline 2 & 3 & 6 \\
\hline 9 & 8 & 7 \\
\hline 10 & 13 & 14 \\
\hline 11 & 12 & 15 \\
\hline 18 & 17 & 16 \\
\hline 19 & 22 & 23 \\
\hline 20 & 21 & $\cdots$ \\
\hline
\end{tabular}
【Solution】According to the problem, we have: The period is $9.2017 \div 9=224 \cdots 1$.
$$
\begin{array}{l}
a=224 \times 3+1=673 ; \\
b=1 . \\
a-b=672 .
\end{array}
$$

Therefore, the answer is: 672