3. Let there be $m$ boys and $n$ girls in the school. Note that the number of boys sitting with girls is equal to the number of girls sitting with boys, i.e., the number $0.4 m (100\% - 60\% = 40\%$ of the number $m)$ is equal to $0.8 n (100\% - 20\% = 80\%$ of $n)$. Therefore, $m = 2n$, and girls make up $\frac{n}{m+n} \cdot 100\% = \frac{n}{2n+n} \cdot 100\% = 33 \frac{1}{3}\%$ of the students.
Answer: $33 \frac{1}{3} \%$.