During heat extraction in enhanced geothermal system (EGS) reservoirs, thermal contraction is induced by cold fluid injection, resulting in thermoelastic deformation. The induced thermoelasticity can alter rock properties, including their fracture aperture and permeability; therefore, the thermoelastic effect is crucial in understanding EGS reservoir behaviour. Based on coupled thermo-hydro-mechanical (THM) processes extended in COMSOL Multiphysics to include the thermoelastic effect, this paper presents a three-dimensional (3D) numerical model of an EGS reservoir with a multiple planar fracture system to investigate the influence of thermoelasticity on reservoir thermal performance. The model is used to perform an in-depth analysis to determine the rate at which the thermoelastic effect develops during heat extraction in relation to a baseline THM model without thermoelasticity. After demonstrating that thermoelasticity influences the thermal performance of EGS reservoirs, the study is further extended to investigate the effect of injection temperature and Young's modulus on fracture aperture opening, reservoir impedance, thermal front propagation, and flow rate. The results show that thermoelasticity affects the long-term thermal performance of EGS reservoirs by reducing the energy extraction rate due to increased flow pathways. Due to high reservoir impedance, the thermoelastic effect appears to cause thermal short circuits (growth of rapidly cooling paths with high flow rates). The results suggest that thermoelasticity has a significant impact on system thermal performance for deep EGS reservoirs.