In the present study, the available methods of predicting the sound transmission loss through infinitely long double-walled cylindrical shells with porous layer are developed to analytically compute the sound transmission loss in triple-walled sandwich cylindrical shells in the presence of an external fluid flow. Loves’ shell theory and Lee’s method based on Biot’s theory are used to describe the motions of thin isotropic triple-walled cylindrical shell and wave propagation in the porous media, respectively. The vibro-acoustic problem for the most complicated configuration of the triple-walled sandwich cylindrical shell is formulated and solved by the transfer matrix method with appropriate boundary conditions. The total transmission loss in a diffuse field is calculated and validated by considering the effect of total internal reflection. Then the transmission loss of triple-walled cylindrical shell is compared with its double-walled counterpart of the same weight. The results generally show a superior performance in sound insulation for the case of triple-walled shell, considerably at mid-high and high frequency regions. Moreover, ten typical configurations, which involve different coupling methods between the walls and porous layers, are considered to completely study the effect of various configurations on the sound transmission properties. As will be shown, a configuration with the largest number of air gaps in its structure provides better performance in sound transmission reduction almost at the entire frequency range. The effects of external fluid flow and azimuthal angle are also studied on the sound transmission loss.