Because underground hydrogen storage offers the potential for large-scale, long-term storage of hydrogen, understanding the adsorption and diffusion behaviors of hydrogen and cushion gas in the reservoir is critical to understanding the underlying mechanisms that control hydrogen storage and transport. Using the molecular simulation method, we investigated the adsorption and diffusion behaviors of H2, CH4 and CO2 in kaolinite slits (10 MPa and 303 K), respectively. The distribution characteristics, excess adsorption amounts, diffusion coefficients and gas-solid interaction energies of three gases in slits were analyzed. Near the pore wall surface, CO2 formed distinct double adsorption layers, CH4 formed a smaller second adsorption layer, and H2 formed a single adsorption layer. The order of excess adsorption amount is CO2>CH4>H2. The rank of diffusivity of gases under the same conditions is H2>CH4>CO2. The interactions between gases and pore walls are in the following order: CO2>CH4>H2. Moreover, the gas-solid interaction is primarily governed by van der Waals interactions. However, H2 and CO2 exhibit significant Coulomb interactions with the pore walls, whereas CH4 has negligible Coulomb interaction with the pore walls. The minerology of the formation results in different charges on the pore surfaces, which has a significant effect on gas storage. This study provides better insights into the mechanisms of hydrogen and cushion gas storage, thus providing a theoretical basis for underground hydrogen storage site selection.