Sound transmission loss through triple-walled sandwich cylindrical shells in the presence of an external mean flow is analytically examined. Love's theory and a simplified method based on Biot's theory are considered to describe the motions of thin isotropic shells and wave propagation in the porous material cores, respectively. The random incidence transmission loss in a diffuse field is calculated numerically by considering the limiting incidence angle due to the total internal reflection. The analytical model and numerical code are validated against both experimental and analytical results reported by previous studies. The transmission loss of triple-walled structure in the diffuse sound field is compared with its double-walled counterpart at the same weight. The results generally show a superior performance in the sound insulation for the triple-walled shell, considerably at mid-high and high frequencies, in comparison with its double-walled counterpart at the same weight. The effects of sandwich shell configuration and air gap depth are also investigated on the sound transmission loss.