This paper proposed a three-dimensional (3-D) instantaneous cutting force (ICF) forecasting model for an indexable disc milling cutter considering cutter run-out. For milling blisk-tunnels, the indexable disc milling cutter must rotates about Y-axis at a certain angle; therefore, the cutting force in Z-direction is related to tangential, radial, and axial components, which have a large influence on processing precision and stability. Herein, a systematic method is presented to effectively determine the 3-D instantaneous cutting force coefficients (ICFCS) through only several calibration experiments. It was found that the distribution laws of the ICFCS match exponent functions of instantaneous uncut chip thickness (IUCT). Experiments were carried out to validate the 3-D cutting force model, it turns out that the presented model can successfully and precisely predict the 3-D ICF of an indexable disc milling cutter under a variety of processing parameters.