An electron's velocity in a Bohr orbit is inversely proportional to the principal quantum number (n):
\( v \propto \frac{1}{n} \)
If the velocity in the first Bohr orbit (n=1) is \(v_1\), and in the third Bohr orbit (n=3) is \(v_3\), the proportionality gives:
\( \frac{v_3}{v_1} = \frac{1/3}{1/1} = \frac{1}{3} \)
Therefore, the ratio \(v_3 : v_1\) is 1:3.