Step 1: According to the Bohr model of the atom, the radius of the \( n \)-th orbit is given by:
\[
r_n = n^2 \cdot r_1,
\]
where \( r_1 \) is the radius of the first orbit.
Step 2: The ratio of the radii of the first three orbits is:
\[
\frac{r_1}{r_1} : \frac{r_2}{r_1} : \frac{r_3}{r_1} = 1 : 4 : 9.
\]
Final Answer:
\[
\boxed{1 : 4 : 9}
\]