A power plant (1) comprising two engine groups (10, 20) and a main power transmission gearbox (2). Each engine group (10, 20) drives the main gearbox (2) mechanically in order to rotate a main rotor (31) of an aircraft (30) at a frequency of rotation NR. A first engine group (10) comprising two main engines (11, 12) is regulated on a first setpoint NR* for the frequency of rotation NR, while a second engine group (20) comprising a secondary engine (21) is regulated on a second setpoint W2* for power. The second setpoint W2* for power is determined so that each secondary engine (21) operates only when a flight power Wvol necessary for the flight of the aircraft (30) is greater than the sum of the main maximum powers WMax1 from each main engine (11, 12).