In Parts I and II, the Hu-Lee-Sum (HLS) correlation was developed and demonstrated to have an inherent generality and simplicity to predict structures I and II gas hydrates suppression temperature for both single and mixed salts systems. Utilization of thermodynamic hydrate inhibitors (THIs) is common practice in industry to prevent hydrate formation. Therefore, the HLS correlation is extended to predict hydrate suppression temperature with either single or mixed THIs, including any combination of inorganic salts and organic inhibitors. It is verified that hydrate equilibrium conditions with organic inhibitors also shows nearly independence of the water activity on temperature. In addition, additive contribution of salts and organic inhibitors to the water activity are used to determine the hydrate suppression temperature. A comprehensive comparison between the literature data and predicted results demonstrate the reliability of the HLS correlation for any combination of inorganic salts and organic inhibitors. (C) 2018 American Institute of Chemical Engineers