Dear friends: I would like to know what is the main function of installing three-way valves on the supply and return lines of the radiator in the heating system? What is the principle?

Three ports, one into the two out, (left into the right and out of the lower) and ordinary valves is different from the bottom of an outlet, when the internal spool in different positions, the outlet is different, such as the spool in the lower part, left and right through, such as when the spool in the upper part of the right outlet is blocked, the left and the bottom of the mouth through. Because the left and right ports are not on a horizontal line. When the high plus emergency de-listing, the valve is closed, the feed water to go bypass.