Three-phase flow involving oil-water two immiscible liquids and gas which is often found in the fields of petroleum production has been studied in this paper. A new method with the combination of a horizontal tube, a downward flow vertical tube and an orifice to measure the flowrates is presented. In this method the frictional pressure drop in the downward vertical tube is replaced by that in the horizontal tube, the void fraction is derived from the gravitational pressure drop, then the volume fraction of the individual phase can also be obtained. The individual flowrates can be calculated when the water fraction is known. This method is applicable for many kinds of oil-wells to measure the flowrates of crude oil, natural gas and water. Compared with other methods, the presented method involves fewer measuring parameters. The experimental results proved quite good accuracy of the method, with measurement deviation within 10%, and reliable results were obtained under high pressure conditions.