Three, (Full marks 12 points) A store sells a product that costs 10 yuan each at 18 yuan each, and can sell 60 units per day. After conducting a market survey, the store manager found that if the selling price of this product (based on 18 yuan each) is increased by 1 yuan, the daily sales volume will decrease by 5 units; if the selling price (based on 18 yuan each) is decreased by 1 yuan, the daily sales volume will increase by 10 units. To maximize daily profit, what should the selling price of this product be set to in yuan per unit?


Three, let the selling price of each item be $x$ yuan, and the daily profit be $s$ yuan. When $x \geqslant 18$, we have
$$
\begin{aligned}
s & =[60-5(x-18)](x-10) \\
& =-5(x-20)^{2}+500 .
\end{aligned}
$$

That is, when the price of the item is increased, the daily profit $s$ is maximized when $x=20$, and the maximum daily profit is 500 yuan. When $x \leqslant 18$, we have
$$
\begin{aligned}
s & =[60+10(18-x)](x-10) \\
& =-10(x-17)^{2}+490 .
\end{aligned}
$$

That is, when the price of the item is decreased, the daily profit is maximized when $x=17$, and the maximum daily profit is 490 yuan.
In summary, the selling price of this item should be set at 20 yuan per item.
Note: (1) This question tests the students' mastery and application of the method of completing the square. If students use the knowledge of quadratic functions to solve the problem correctly, full marks will also be given.
(2) This question must be solved by considering two different scenarios. If only one scenario is explained, or if only one scenario is correct, a maximum of 7 points will be awarded.