This paper reports the three-phase (ice + hydrate + guest-rich vapor) equilibrium pressure-temperature conditions at temperatures (243 to 272) K in the systems of water and each of the following two tertiary gas mixtures of methane, ethane, and propane: 90:7:3 molar ratio and 99.48:0.5:0.02 molar ratio. The former is a Simulated natural gas (natural-gas composition). The hatter has a methane-rich composition that is equal to that of the vapor phase in equilibrium with the clathrate hydrate that has the guest composition of 90:7:3 molar ratio. The pressure ranges of the present measurements in the two systems are (0.233 to 0.711) MPa in the natural-gas-composition gas mixture system and (0.939 to 2.070) MPa in the methane-rich-composition gas mixture system. The measurements were carried Out using the batch, isochoric procedure. The measured data were compared with the corresponding predictions using phase equilibrium calculation programs.