A three-phase body consisting of microemulsion, water, and oil phases is distorted in a rectangular phase diagram, when two surfactants are mixed to adjust the hydrophile-lipophile balance (HLB) of the mixture in a given water/oil system. This distortion is mainly attributed to the difference in solubilities of each surfactant in three coexisting phases. The HLB plane method is used to understand the whole three-phase behavior considering the mixing fraction of surfactant at the microscopic water/oil interface inside the microemulsion phase. With this method, the position of a particular three-phase tie triangle is calculated in the space of compositions. We have modified the HLB plane method to be applicable for the three-phase behavior of nonionic surfactant-small amphiphile systems, in which small amphiphiles are considerably soluble both in water and oil. The method can be used to accurately determine the HLB plane (three-phase tie triangle) in phase diagrams. The three-phase behavior in polyoxyethylene dodecyl ether-small amphiphile (2-butoxyethanol, 2-isobutoxyethanol, or propanol) systems is well analyzed by the new method.