A simulation study -has been made of the three-component NO-Co-O2 heterogeneous catalytic reaction, where the three species are adsorbed on a hexagonal lattice (each site having six nearest neighbours (nn)). NO and O2 are considered to dissociate on adsorption while CO is adsorbed in its molecular form. Nearest adsorbed CO and O react to form CO2, while N combines with the nearest N to form N2. The system is studied for different ratios of the feed concentrations, which is varied to keep the total concentration (y) normalised to unity, i. e., y(CO) + y(O) + y(NO) = 1. Different phase transitions and respective stationary states are studied by Monte Carlo (MC) simulation. Many interesting observations are presented. The most interesting outcome of the present study is that the steady reactive state in NO-Co-O2 reaction has a much larger range than in individual two-component NO-CO and CO-O2 reactions.