First-principle calculations and kinetic Monte-Carlo simulations were performed to study the hydrogen storage properties of the intermetallic hydrides MgNiH3, MgCoH3 and thier mixture namely MgCo0.5Ni0.5H3. Based on the heat of formation, desoprtion temperature, activation energies computed from DFT calculations and KMC simulations, we show that the MgNiH3 involves a fast kinetic while it is thermodynamically unstable (-9.96 kJ/mol.H-2; 76.61 K) whereas MgCoH3 has a high thermodynamic stability (-73.32 kJ/mol.H-2; 560.97 K) which prevents their application for mobile hydrogen storage. On the other hand, the electronic structures show that the Ni weakens the strong Co-H bonding of the mixture MgCo0.5Ni0.5H3, which enhances significantly its stability and its desorption temperature (-45.92 kJ/mol.H-2 and 351.33 K) without reducing its high volumetric capacity 133.73 g.H-2/1. Kinetic Monte-Carlo simulations show that MgCo0.5Ni0.5H3 exhibits a fast charging time (only 4.6 min at 400 K and 10 bar). Thermodynamic properties including entropy S, Gibbs free energy G and thermal expansion coefficient are predicted within the quasi-harmonic approximation. It is verified that crystal structure of MgCo0.5Ni0.5H3 is stable. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.