A laser flash tri-layered analysis was conducted to measure the thermal diffusivity of nickel-based superalloy Inconel 718 (IN718) powder, Ti64 powder, and 316L stainless steel powder, which are widely used in laser powder bed fusion (LPBF) additive manufacturing. In the LPBF process, the thermal properties of the powder bed are strongly influenced by various input parameters. Understanding these thermal transport properties is essential for predicting melt pool behavior, microstructural evolution, and the final part quality. In this study, the thermal diffusivities of the powder samples were measured in two distinct gaseous environments, helium (He) and nitrogen (N₂), inside a high-temperature furnace. Measurements were performed at 200 °C, 400 °C, and 600 °C to investigate the combined effects of gas environment and temperature. The results indicate that variations in temperature have only a minimal effect on thermal diffusivity, whereas the surrounding gas environment plays a critical role. Helium consistently enhanced the thermal diffusivity compared to nitrogen. For comparison, the thermal diffusivity of solid samples of IN718, Ti64, and 316L stainless steel was also measured. Additionally, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses were conducted to examine changes in powder morphology and surface oxidation state after exposure to high temperature and different gas atmospheres.