As Part III, following the authors’ previous studies, the aerodynamic performance of two different tandem LSDs (low solidity diffusers), Tandem (A) and (B), in a high-pressure centrifugal compressor was numerically investigated over flow rates from impeller choke to minimal flows available in computation. Tandem (A) was of conventional design where the first row came directly from the authors’ previous studies (Part I Oh and Agrawal, 2007, “Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor - Part I : Influence of Vane Solidity,” ASME Paper No. GT2007-27260, and Part II: Oh , 2008, “Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor - Part II : Influence of Vane Stagger,” ASME Paper No. GT2008-50178) selected as the highest efficiency vane at design flow, and the second row was designed to be added considering flow conditions at the exit of the first row vane. Tandem (B) followed a creative patent-pending concept where the number of the first row vanes was doubled with much smaller vane chord keeping a low solidity. A position parameter of RCP (relative circumferential position) was introduced to see the effect of the relative location of the second row vane. Using an in-house Navier-Stokes solver with finite volume time marching methods, overall performance was predicted to be compared with each other. Detailed investigation on the behavior of the static pressure recovery and the total pressure loss coefficient in both diffuser designs helps determine why Tandem (A) design is better and the case of RCP = 0.3 gives the best performance.