This study presents a three-dimensional fully coupled thermo-hydro-mechanical (THM) numerical analysis of the response of Opalinus Clay to thermal loading, using the Imperial College Geomechanics Toolkit, a finite element-based simulator developed for modelling coupled processes in geomaterials. The analysis incorporates a constitutive framework that accounts for thermo-poro-elastic deformation and the differential thermal expansion between the fluid and solid phases that generates excess pore pressures in low-permeability media. The HE-D in situ heating test, conducted in the Opalinus Clay at the Mont Terri Underground Rock Laboratory, is employed as a case study to investigate the key THM processes expected to occur around geological disposal facilities (GDFs) for heat-producing radioactive waste [1-3]. The numerical model is calibrated against the observed temperature evolution, pore pressure generation, and hydromechanical responses during the heating and cooling phases of the experiment. Three-dimensional simulations enable the incorporation of in situ stresses and material parameters, providing a structured framework for interpreting the integrated THM behaviour of this stiff, sedimentary clay. The effects of material heterogeneities, including variations in permeability, stiffness, and thermal properties, are systematically investigated through sensitivity analyses to quantify their influence on pore pressure build-up, effective stress changes, and potential for thermally-induced responses near excavations. This work aims to demonstrate how excess pore pressures can reduce mean effective stresses during heating, with heterogeneities potentially affecting localised behaviour and the time scale of pressure dissipation due to the very low permeability of the clay. Overall, the simulations provide insights into the role of key parameters and heterogeneities in the THM processes observed in the HE-D test, which are representative of those anticipated in the near field of a GDF, contributing to improved understanding of host rock performance for the safe long-term containment of high-level radioactive waste.