In Chinese medicine the Triple Burner (Triple Warmer, Triple Heater) does not correspond to any exact organ in the body. It is best understood as the functional relationship between various Organs that regulate Water. These are mainly the lung, spleen and kidney, but also include the small Intestine and the bladder.
Even though the Triple warmer in TCM has its own meridian and is regarded as an organ function, it refers to the coordination of several organs and physiological functions with each other. The Upper Warmer refers to all the organs and parts from the base of the sternum upwards; the Middle Warmer includes the organs and parts in the middle region from the navel to the base of the sternum; the Lower Warmer refers to all the organs and parts from the navel downwards. Interestingly, even though the physical location of the Liver would be in the Middle Warmer, it is actually considered a Lower Warmer organ.
San-jiao is the "three burning cavities", and so the Triple Burner carries responsibilities for regulation of temperature in the body (hypothalymus) and for the coordination of the functions of the chest (for breathing), stomach (for digestion) and pelvis (for excretion and reproduction); i.e. for the coordination of respiration, circulation, digestion, elimination and sexuality.
The Triple Burner, through its fiery nature, regulates the balance and movement of water: the Upper Burner is "vaporized water", or "mist", in the upper body (or lung); the Middle Burner is a "foam" corresponding to the water in the stomach and spleen; the Lower Burner is a "swamp", corresponding to the impurer Fluids of the Lower Body (the kidney, small intestine and large intestine, and bladder).
The small intestine is also an Organ of Fire Yang, and is paired with its Yin Organ, the Heart, and the Triple Warmer with the Pericardium.