This study investigates the influence of correlations between particle morphology and gradation on the critical shear strength of three-dimensional granular assemblies via numerical simulations. While grain shape is acknowledged to play a central role in the mechanical behavior of granular media, only a few works have explored the combined effects of grain shapes varying with grain size. Employing three-dimensional discreteelement simulations, we explore the shear behavior of samples with diverse particle size distributions, where grain shapes (ranging from spheres to very angular polyhedra) are assigned based on the relative size of each particle. Using drained triaxial shear tests, we analyze the macroscopic behavior up to large deformation levels. Micro-mechanical analyses are also conducted to understand the underlying mechanisms governing the observed macroscopic behavior, highlighting the role of grain connectivity and force transmission. Surprisingly, well graded samples composed of coarse angular grains and fine rounded ones are not capable of developing higher shear strengths than uniform samples with the same diversity on grain shapes. Conversely, samples where coarser grains are rounded and fines are angular, show constant shear strength as the particle size distribution becomes broader. These findings underscore the importance of simultaneously considering grain size and shape distributions for the assessment of realistic granular soil behavior.