<p>High confining pressure and hydraulic pressure are key factors that limit the efficiency of rock fragmentation in deep formations. Understanding the mechanisms of rock failure under confining and hydraulic pressures is crucial for improving rock-breaking efficiency. While 2D models fail to fully characterize cuttings morphology and crack propagation under hydraulic pressure, 3D models remain insufficient. This study uses a 3D discrete element model to establish a single-cutter cutting model for heterogeneous granite under confining and hydraulic pressures. It systematically examines the effects of cutting depth, cutter dip angle, confining pressure, and hydraulic pressure on rock-cutting performance, from perspectives such as crack propagation, debris distribution, energy dissipation, and rock-breaking efficiency. The results show that under confining and hydraulic pressures, rock failure primarily occurs via tensile failure. Crack propagation in the cutting direction is suppressed, while vertical crack propagation is enhanced, increasing the roughness of the cutting groove. The rock debris mainly consists of small-sized rock fragments, with reduced cutting depth and increased pressure further raising the proportion of smaller debris. Most energy during the rock-cutting process dissipates as friction, and mechanical specific energy (MSE) increases with the proportion of plastic energy. Additionally, under confining and hydraulic pressures, plastic fragmentation dominates rock failure, with reduced cutting depth, increased cutter dip angle, and higher pressure leading to a greater proportion of plastic fragmentation. This study provides valuable theoretical support for the design and optimization of PDC drill bits.</p>