The United Kingdom has been at the forefront of testing the feasibility of hot dry rock (HDR) geothermal energy resources since the mid-1970s. Several experiments were carried out between 1977 to 1991 at the Rosemanowes site in Cornwall. After a long period of testing the project was discontinued in 1991 due to some challenges. However, investors are back on board, developing a deep enhanced geothermal system (EGS) at the United Downs in Cornwall, Southwest England. The project aims to tap the abundant thermal energy underneath Cornish granites. So far, drilling of wellbores have been completed as of June 2019, and notable progress has been made on testing. Two features make the United Downs Deep Geothermal (UDDGP) project novel: a larger wellbore separation distance of about 2,882 m and the intersection of a fault zone by both wellbores. Thus, this paper presents a new three-dimensional (3D) model based on the site's limited data to predict system performance. The model implements the coupled effect of thermo-hydro-mechanical (THM) processes using the COMSOL Multiphysics. The simulation results show that the model prediction ability agrees with the existing EGS behaviour. However, more rigorous site data is required for an in-depth analysis of the system performance.