3. The quality requirements of a product are divided into four different levels from low to high, labeled as $1,2,3,4$. If the working hours remain unchanged, the workshop can produce 40 units of the lowest level (i.e., level 1) product per day, with a profit of 16 yuan per unit; if the level is increased by one, the profit per unit increases by 1 yuan, but the daily production decreases by 2 units. Now the workshop plans to produce only one level of product. To maximize profit, the workshop should produce level $\qquad$ product.
3.3.

Let the profit obtained from producing products of the $x$-th grade in the workshop be $y$. According to the problem, we have
$$
\begin{array}{l}
y=[40-2(x-1)][16+(x-1)] \\
=-2 x^{2}+12 x+630=-2(x-3)^{2}+648 .
\end{array}
$$

Therefore, when $x=3$, the profit $y$ is maximized, at 648 yuan.