[Objective] Under the actual filling conditions of high rockfill dams, rockfill materials are typically subjected to complex three-dimensional stress states. This study aims to investigate the effects of spherical stress (p), intermediate principal stress coefficient (b), and initial dry density ( ρ₀) on the stress-strain relationships, strength characteristics, and non-coaxiality of stress-strain increment directions on the π-plane of rockfill materials. [Methods] Consolidated-drained true triaxial shear tests were conducted on typical rockfill materials for dam construction with constant p and constant b stress paths under three-dimensional conditions involving different p, b, and ρ₀. Based on experimentally measured data, conventional strength criteria considering intermediate principal stress effects were comparatively analyzed for their applicability to rockfill material strength. [Results] The results showed that: (1) as p increased, the q- ε₁ relationship curves exhibited a progressively steeper strain-hardening trend. The initial shear modulus (E_i) increased correspondingly, and peak shear strength (q_max) significantly enhanced. Shear contraction grew while dilation development was suppressed, and the failure stress ratio (M_b) gradually decreased as p increased. (2) With increasing b, both E_i and q_max progressively declined. Reduced shear contraction made the rockfill materials transition more rapidly into dilation, with increasingly significant dilatancy. M_b demonstrated a downward trend. (3) As ρ₀ increased, both E_i and q_max increased markedly. Rockfill materials entered dilation earlier with progressively greater dilatancy, and M_b exhibited an upward trend. [Conclusion] The conclusions are as follows: (1) an increase in p significantly enhances the shear strength of rockfill materials, and the influence of b on its strength progressively diminishes. As ρ₀ rises, both the initial shear modulus and shear strength increase markedly, highlighting the necessity for strict quality control during dam construction. (2) The Lade-Duncan strength criterion effectively characterizes the nonlinear strength characteristics of rockfill materials, while the Mohr-Coulomb criterion yields conservative predictions due to its neglect of intermediate principal stress effects. (3) Non-coaxiality between stress and strain increment directions is observed on the π-plane during testing. This non-coaxial behavior is most pronounced during the initial shear stage, and it gradually transitions toward coaxiality as the specimen approaches instability failure. (4) The non-coaxiality of rockfill materials initially increases and then decreases with increasing b. It gradually weakens with increasing p, with the weakening rate diminishing, and it intensifies with higher ρ₀ with a progressively faster intensifying rate.