The number of de Broglie waves associated with the electron in its \( n \)-th orbit is given by: \[ n = \frac{2 \pi r}{\lambda} \] where \( r \) is the radius of the orbit and \( \lambda \) is the de Broglie wavelength of the electron. For the third orbit (\( n = 3 \)), we have 3 complete de Broglie waves.
Hence, the correct answer is (B).