For three-component systems composed of a crystalline polymer (component 3), solvent (component 2) and non-solvent (component 1), polymer-poor liquid-polymer-rich liquid and liquid-polymer crystal phase equilibrium lines are constructed. When the two lines overlap, four regions of different phase separations are distinguished. For these systems, the Gibbs free energy change of mixing surface is constructed and the equilibrium line is then a collection of points where a common tangent plane touches the Gibbs free energy change of mixing surface. Based on the Flory-Huggins theory, computer programs are developed for phase diagram construction and the Gibbs free energy change of mixing surface, and the diagrams and Gibbs free energy change of mixing surface for the poly(vinylidene fluoride)/dimethylformamide/octanol system are presented.