Since part-through crack growth stages occupy most of crack growth life of engineering structures, it is essential to investigate the fracture parameters of part-through cracks. However, the complex three-dimensional (3D) stress states make it difficult to efficiently dominate the crack-tip fields. Here, the 3D elastic–plastic stress intensity factor Kδ-Tz is extended to dominate part-through cracks. Systematic 3D finite element (FE) analyses are conducted for typical part-through cracks (embedded, corner, and surface cracks) considering different elliptical ratios and hardening exponents. It is found that the predicted stress distributions by the δ-Tz solution agree well with 3D FE results. Additionally, the predictive performance of the δ-Tz solution improves with increasing hardening exponents. Across all experimental and numerical results, the variation of J-integral along the crack front line can reach 200%, while remaining within 21% for Kδ-Tz. These results demonstrate that Kδ-Tz can reduce geometric constraints effectively and be a more stable elastic–plastic fracture parameter for part-through cracks in engineering structures.