Suppose that there are $n$ employees at Sergio's store. After his first average calculation, his $n$ employees had sold an average of 75 items each, which means that a total of $75n$ items had been sold. The next day, one employee sold 6 items, one sold 5, one sold 4, and the remaining $(n-3)$ employees each sold 3 items. After this day, the total number of items sold to date was $75n+(6+5+4+(n-3)3)$ or $75n+15+3n-9$ or $78n+6$. Since the new average number of items sold per employee was 78.3, then $\frac{78n+6}{n}=78.3$ or $78n+6=78.3n$. Therefore, $0.3n=6$ or $n=20$. Thus, there are 20 employees in the store.