A method comprising: removing heat from a refrigerant using a high side heat exchanger; storing the refrigerant from the high side heat exchanger using a flash tank; discharging a flash gas from the flash tank; removing heat from a first space proximate a first low temperature load using the refrigerant; removing heat from a second space proximate a second low temperature load using the refrigerant; removing heat from a third space proximate a medium temperature load using the refrigerant; compressing the refrigerant from the first low temperature load using a first low temperature compressor; compressing the refrigerant from the second low temperature load using a second low temperature compressor; compressing the refrigerant from the medium temperature load and the refrigerant from the second low temperature compressor using a medium temperature compressor; when a parallel compressor is on: directing, by a first valve, the refrigerant from the first low temperature compressor to the parallel compressor and away from the medium temperature compressor; and compressing the refrigerant from the first low temperature compressor and the flash gas using the parallel compressor; when the parallel compressor is off: directing, by the first valve, the refrigerant from the first low temperature compressor to the medium temperature compressor and away from the parallel compressor; directing, by a second valve, the flash gas from the flash tank to the medium temperature compressor and away from parallel compressor; and compressing, by the medium temperature compressor, the refrigerant from the first low temperature compressor and the flash gas. removing heat from a refrigerant using a high side heat exchanger; storing the refrigerant from the high side heat exchanger using a flash tank; discharging a flash gas from the flash tank; removing heat from a first space proximate a first low temperature load using the refrigerant; removing heat from a second space proximate a second low temperature load using the refrigerant; removing heat from a third space proximate a medium temperature load using the refrigerant; compressing the refrigerant from the first low temperature load using a first low temperature compressor; compressing the refrigerant from the second low temperature load using a second low temperature compressor; compressing the refrigerant from the medium temperature load and the refrigerant from the second low temperature compressor using a medium temperature compressor; when a parallel compressor is on: directing, by a first valve, the refrigerant from the first low temperature compressor to the parallel compressor and away from the medium temperature compressor; and compressing the refrigerant from the first low temperature compressor and the flash gas using the parallel compressor; directing, by a first valve, the refrigerant from the first low temperature compressor to the parallel compressor and away from the medium temperature compressor; and compressing the refrigerant from the first low temperature compressor and the flash gas using the parallel compressor; when the parallel compressor is off: directing, by the first valve, the refrigerant from the first low temperature compressor to the medium temperature compressor and away from the parallel compressor; directing, by a second valve, the flash gas from the flash tank to the medium temperature compressor and away from parallel compressor; and compressing, by the medium temperature compressor, the refrigerant from the first low temperature compressor and the flash gas. directing, by the first valve, the refrigerant from the first low temperature compressor to the medium temperature compressor and away from the parallel compressor; directing, by a second valve, the flash gas from the flash tank to the medium temperature compressor and away from parallel compressor; and compressing, by the medium temperature compressor, the refrigerant from the first low temperature compressor and the flash gas.