Let the number of girls be \(3x\) and the number of boys be \(x\). According to the problem, the total number of students is given as:
\[ 3x + x = 400 \]
Combining the terms gives:
\[ 4x = 400 \]
To find \(x\), divide both sides by 4:
\[ x = \frac{400}{4} = 100 \]
Thus, the number of boys is:
\[ x = 100 \]
Therefore, there are 100 boys in the school.