In this work we present a systematic study of the three-dimensional extension of the ring dark soliton examining its existence, stability, and dynamics in isotropic harmonically trapped Bose-Einstein condensates. Detuning the chemical potential from the linear limit, the ring dark soliton becomes unstable immediately, but can be fully stabilized by an external cylindrical potential. The ring has a large number of unstable modes which are analyzed through spectral stability analysis. Furthermore, a few typical destabilization dynamical scenarios are revealed with a number of interesting vortical structures emerging such as the two or four coaxial parallel vortex rings. In the process of considering the stability of the structure, we also develop a modified version of the degenerate perturbation theory method for characterizing the spectra of the coherent structure. This semianalytical method can be reliably applied to any soliton with a linear limit to explore its spectral properties near this limit. The good agreement of the resulting spectrum is illustrated via a comparison with the full numerical Bogolyubov-de Gennes spectrum. The application of the method to the two-component ring dark-bright soliton is also discussed. * Electronic address: wenlongcmp@gmail.com † Electronic address: kevrekid@math.umass.edu FIG. 1: Density contour and phase profile of a stationary ring dark soliton in a spherical harmonic trap of frequency ω = 1 at chemical potential µ = 16 (Eq. (1)). There is a ring of vanishing density separating two segments with phase difference of π shown as two different colours (red and green). Note the ring is not vertically straight, but bends. The state is generated from numerical computation. the incongruence between the cylindrical symmetry of the 2D pattern and the closer to spherical (or more accurately: ellipsoidal) nature of the 3D condensate. A cross section perpendicular to the axis is a 2D ring, and the radius is larger at the edges and smaller at the center. See Fig. 1 for a density and phase profile of the RDS.Despite its extensive analysis in the 2D case, including the recent corroboration of its modes of vibration and even its multi-component extension [18], the RDS has not been studied in detail in three dimensions. Instead, other structures, such as the planar dark soliton (PDS) [19,20] and the spherical dark soliton (SDS) [20,21] have been considered due to their symmetry. The latter have been dynamically recognized as generating spontaneously structures such as vortex rings and multi-ring generalizations thereof [19,22,23]. In that arXiv:1910.02272v1 [cond-mat.quant-gas]