For a three-level atom,two nondegenerate (even microwave and optical) electric dipole transitions are usually al-lowed;for either of these,the fluorescence spectra are well-described in terms of spontaneous transitions from a triplet of dressed sublevels to an adjacent lower-lying triplet.When the three dressed sublevels are equally spaced from each other,a remarkable feature known as degenerate cascade fluorescence takes place,which displays a five-peaked structure.We show that a single cavity can make all the spectral lines extremely narrow,whether they arise from cavity-coupled or cavity-free transitions.This effect is based on intrinsic cascade lasing feedback and makes it possible to use a single microwave cavity(even a bad cavity) to narrow the spectral lines in the optical frequency regime.