Depicting the co-occurrence of erosion and side slope failure during landslide dam breaches using numerical models remains a challenge. In this study, a 3D Smoothed Particle Hydrodynamics (SPH) framework is proposed by coupling the erosion and side slope failure processes. Erosion downcutting is depicted by calculating the washed-away materials due to shear forces exerted by overtopping flow, while side slope failure is simulated using the Drucker-Prager model. In the computation domain, the destabilized slope soil particles are identified, then set to deform like fluid, and subsequently mixed with the flow. The proposed model achieves a refined 3D simulation of breach channel morphology evolution. According to the driving factors, the breaching process can be divided into two stages: the erosion-dominated breach deepening stage, and the simultaneous deepening and widening stage driven by both erosion and side slope failure. Dam erodibility affects the rate of breaching, while soil cohesion influences the breach channel evolution. As soil erosion rate increases, peak discharge increases following a power function, while peak timing decreases according to a power function. As soil cohesion increases, side slope failure slows, resulting in steeper slopes and more pronounced “headcut” erosion, which shifts the slope failure mode from sliding to collapsing.