A device for partially deactivating the cylinders of an internal combustion engine having three pistons which are connected to a crank shaft, which are each mounted in an associated cylinder and which each collaborate with at least one inlet valve moved by a first cam shaft and an exhaust valve moved by a second cam shaft, the device comprising a means of partial deactivation which is designed to deactivate the three cylinders alternately in a uniform fashion, and being designed to occupy a state of activation of the cylinders in which the partial deactivation means are deactivated, and a state of partial deactivation of the cylinders in which the partial deactivation means are activated, characterized in that the first cam shaft and the second cam shaft are rotationally connected to the crank shaft in a 1-to-4 reduction ratio, each cam shaft is mounted slidably axially between a first activation position in which the device occupies its state of activation of the cylinders and a second position of partial deactivation in which the device occupies its state of partial deactivation of the cylinders, the first cam shaft and the second cam shaft each having a hollow cylindrical shape which delimits an axial bore, and in that the device comprises a first axial rod and a second axial rod which are rotationally connected to the first cam shaft and to the second cam shaft respectively, each rod being arranged in the bore of the associated cam shaft such that each cam shaft is mounted slidably axially in said associated rod between its position of activation and its position of partial deactivation, and each cam shaft comprising: - at least a first set of double cams which is designed to actuate the valves associated with each cylinder twice per turn of the associated cam shaft, such that each cylinder is activated, - at least a second set of simple cams or intermediate double cams (36a, 36b) forming partial deactivation means, which is designed to actuate the valves associated with each cylinder once per turn of the associated cam shaft, such that each cylinder is partially deactivated to prevent every other explosion, and in that the first set of double cams actuates the associated valves when the device occupies its state of activation of the cylinders, and the second set of simple cams or intermediate double cams (36a, 36b) actuates the associated valves when the device occupies its state of partial deactivation. - at least a first set of double cams which is designed to actuate the valves associated with each cylinder twice per turn of the associated cam shaft, such that each cylinder is activated, - at least a second set of simple cams or intermediate double cams (36a, 36b) forming partial deactivation means, which is designed to actuate the valves associated with each cylinder once per turn of the associated cam shaft, such that each cylinder is partially deactivated to prevent every other explosion, and in that the first set of double cams actuates the associated valves when the device occupies its state of activation of the cylinders, and the second set of simple cams or intermediate double cams (36a, 36b) actuates the associated valves when the device occupies its state of partial deactivation.