Strong spectral-mode-selection behavior has been observed in a three-mirror solid-state resonator that was constructed by colinearly aligning an aspheric feedback mirror with a standing-wave two-mirror resonator. This spectral mode selection is measured to be much stronger than that predicted by the frequency selection theory of simple Fabry-Perot cavities. Our study shows that the spatial modes and their associated eigenvalues vary at different oscillation wavelengths in a complex extended-cavity resonator. It is the interplay between spatial modes and spectral modes that induces the strong spectral-mode selection