is a term used in rocketry, it's kinda like bipropellent
except three different tanks of chemicals are mixed. Generally it's not worth the hassle, but is used occasionally.
The advantage of it is that the mixture of the chemicals can be continually modified- early on in a launch for example, dense fuels should be burnt to get rid of mass as soon as possible, but they generally give less exhaust velocity but the higher density allows for more thrust. Later on in the flight the rocket is lighter and fighting gravity less- there the higher exhaust velocity that is given by hydrogen is more useful.
At each altitude a different mixture of kerosene, hydrogen and LOX is optimum, and the ratios can be varied throughout the burn, and this is one of the main advantages this technique gives.