The phase-field method provides an effective framework for modeling fracture in high-temperature brittle rocks due to its capability in describing multi-physics coupling[1] and mixed-mode cracking[2]. In this work, a three-dimensional thermo-mechanically coupled mixed phase-field model for heterogeneous granite is developed to investigate fracture behavior under high-temperature true triaxial compression, motivated by wellbore instability issues in geothermal energy applications. The formulation combines a mixed-mode phase-field fracture approach with a three-dimensional rock strength criterion, allowing for the distinction between tensile- and shear-dominated fractures. Temperature-dependent mechanical and thermal properties are introduced based on laboratory measurements, while material heterogeneity is represented through a Voronoi tessellation. The thermo-mechanical coupling is verified using a thick-walled cylinder heat-transfer benchmark. Numerical results demonstrate that the proposed model is able to reproduce the evolution of temperature fields and the initiation of randomly distributed tensile cracks during heating. Under true triaxial loading conditions, the model captures the strength, deformation response, and fracture propagation characteristics of brittle rocks, as well as the experimentally observed degradation of strength with increasing temperature. The proposed approach offers a robust numerical tool for analyzing three-dimensional fracture mechanisms in heterogeneous rocks subjected to coupled thermo-mechanical loading.