3. If the three-digit decimal number $n=abc$ satisfies that $a$, $b$, $c$ form an arithmetic sequence, then the maximum possible value of a prime factor of $n$ is $\qquad$
3. 317 .
$$
\begin{array}{l}
31 n, 999=27 \times 37, 987=3 \times 7 \times 47, \\
963=9 \times 107, 951=3 \times 317 .
\end{array}
$$