Using the free-water assumption and a modified Rachford-Rice (MRR) equation proposed in this work, a novel method is developed to calculate the three-phase flash of water-hydrocarbon mixtures. In the method, the number of phases can be determined instantly, the computation is as fast as that of two-phase flash, and binary parameters between water and hydrocarbons are only optional. The efficiency of the developed method is demonstrated by its reduction to the accurate Scheffer equation, its good results for two real systems, and its favorable comparison with regular three-phase flash.