Answer
$a_n=a_{n-2}^2+a_{n-1}$
Work Step by Step
We study the terms of the given sequence and notice the pattern:
$$\begin{align*}
a_1&=3\\
a_2&=8\\
a_3&=3^2+8=17\\
a_4&=8^2+17=81\\
a_5&=17^2+81=370.
\end{align*}$$
The recursive rule is:
$$a_n=a_{n-2}^2+a_{n-1}.$$