The Wiebe function is a simple and cost-effective analytical approach to approximate the burn rates in internal combustion (IC) engines. Previous studies indicated that a double-Wiebe function model can better describe the two-stage combustion process inside diesel engines retrofitted to natural gas (NG) spark ignition (SI) compared with a single-Wiebe function. Specifically, the two Wiebe functions are associated with the bowl burn and the squish burn. However, the long tail in the energy release at the end of combustion produces some differences between experiment and model, which can be attributed to the complexity of the late oxidation process inside the post-flame zone and the incomplete combustion of the unburned mixture flowing out from engine crevices. To improve the matching between the model and experimental data, this paper investigated the effect of adding a third Wiebe function just to describe the long tail in the energy release at the end of combustion. The results indicated that such a methodology greatly improved the fitting accuracy in terms of phasing and magnitude of the heat release rate in each combustion stage.