3. Let there be $x$ seventh-graders participating in the tournament, who together scored $n$ points. Then the number of eighth-graders participating in the tournament is $10 * x$ people, and the total points they scored is $4.5 * n$ points. Therefore, a total of $11 * x$ students participated in the tournament, and they collectively scored $5.5 * n$ points. The total number of points scored by all participants is equal to the number of games played. We get the equation:

$$
\begin{gathered}
5.5 * n = \frac{11 * x * (11 x - 1)}{2} \\
n = x * (11 x - 1)
\end{gathered}
$$

Each seventh-grader played $11 x - 1$ games (since there are a total of $11 x$ participants). Therefore, $x$ seventh-graders could collectively score $n$ points, i.e., $x * (11 x - 1)$ points, only if each of them won all their games. This is only possible when $x = 1$ (two seventh-graders cannot simultaneously win against each other).
Answer: 1 student from 7th grade participated in the tournament and scored 10 points.