Three, in a city, there are $n$ high schools, the $i$-th high school sends $C_{i}$ students $\left(1 \leqslant C_{i} \leqslant 39\right.$, $1 \leqslant i \leqslant n$ ) to the gym to watch a ball game, the total number of all students is $\sum_{i=1}^{n} C_{i}=1990$. Each row in the stands has 199 seats. It is required that students from the same school must sit in the same row. How many rows does the gym need to arrange at least to ensure that all students can be seated?
Three, the conclusion is that at least 12 rows are needed to ensure all students can be seated.
Number the middle schools, and let the students from School No. 1 sit in the first row, followed by the students from School No. 2, ..., and so on, with students from each school sitting together. When a row is full, if there are still students from the same school, allow those remaining students to sit at the beginning of the next row. Since each row has 199 seats and the total number of students is 1990, after 10 rows, all students will be seated. Then, adjust the students at the breaks, i.e., if the end of the first row and the beginning of the second row are occupied by students from the same school, move all students from that school out, ..., this way, at most 9 schools' students will be moved out. Since each school has at most 39 students, and $5 \times 39=195<199$, place the first 5 schools' students in the 11th row, and the remaining 4 schools' students in the 12th row. Thus, it is proven that arranging 12 rows can ensure all students can be seated and meet the requirement: students from the same school only sit in the same row.

If only 11 rows are arranged, sometimes it is indeed not enough. For example, suppose $n=59$, with 58 middle schools each sending 34 students, and 1 school sending 18 students, then $58 \times 34+18=1990$. Since $5 \times 34=170<199<204=6 \times 34$, one row can seat 6 schools' students, while the other 10 rows can each seat 5 schools' students. Therefore, 11 rows can seat at most $6+5 \times 10=56$ schools' students, leaving two schools' students without seats. This shows that 12 rows are still the minimum number of rows needed to ensure all students can be seated according to the rules.