Invariant forms of the flame stretch on three dimensional flame surfaces were derived by M. Matalon (1983), S. H. Chung and C. K. Law (1984), and S. M. Candel and T. J. Poinsot (1990) respectively in different forms. To resolve potential confusions, the three model forms were compared, and their mutual identities were described in this study. The respective model equations were also organized in their decomposed expressions, by normal and tangential stretch components and by components induced by flow velocity and flame speed. Their applications into numerical and experimental data were discussed, and potential application issues were addressed by illustrating flame stretch calculation procedures. In most situations, necessary parameters (e.g., flame location, migration velocity, flame speed, flow velocity, etc.) could be extracted from the data on the flame surface. Some model equations require operations covering outside of the flame surface, which could be unknowns to calculate flame stretch.