Field practices have demonstrated that three-dimensional well-pad development technology can effectively exploit untapped reserves of shale oil/gas resources. However, the applications of the three-dimensional well-pad development has led to increased horizontal/vertical multi-well interferences. This phenomenon has become increasingly severe, which restricts the efficient development of shale oil and gas resources. Thus, accurately assessing the degree of well interference and its influencing factors is a critical issue that urgently needs to be addressed. To tackle this, we proposed a three-dimensional numerical model of multi-well interference. The proposed model is based on the numerical well-testing model for fractured reservoirs based on unstructured tetrahedral mesh. In addition, the discrete fracture model (DFM) is used to account for non-penetrating geological fracture features. The numerical model is solved using Newton-Raphson iteration. Its accuracy is verified by comparing it with a commercial well testing software. After verification, a coefficient for the degree of well interference is defined, and the analyses of well interference under different well spacings and fracture parameters are conducted. The results indicate that increasing the horizontal spacing leads to a rapid decrease in the degree of well interference. As the vertical spacing increases, the degree of well interference initially increases and then decreases. The influence of horizontal spacing on the degree of well interference is more significant compared to the impact of vertical spacing. The degree of interference increases with the greater production intensity, hydraulic fracture number, hydraulic fracture length, and hydraulic fracture conductivity of adjacent well. The existences of natural fractures can strengthen the degree of well interference. Applying grey relational analysis method, the correlation between the degree of well interference and various influencing parameters was calculated for the well pads from the Ordos Basin. The main controlling factors affecting the degree of well interference are including the horizontal spacing along the fracture direction, fracture length, and fracture number. It is also found that the vertical spacing between wells has a significant impact on the degree of well interference, which should not be underestimated. A reasonable vertical spacing can effectively reduce the degree of well interference. The research results can provide a theoretical basis for the evaluation of well interference and the optimization of development parameters in shale oil development.