Achieving near-unity nonlinear circular dichroism (CD) typically requires geometric symmetry breaking, making it challenging to combine helicity selectivity, resonant enhancement, and structural simplicity. We show that crystal orientation enables near-unity third-harmonic (TH) CD in highly symmetric resonant metasurfaces formed by a square lattice of circular holes in a cubic nonlinear material. Under circularly polarized excitation, strong near-field components of both helicities enabled by the rotational symmetry of the metasurface open two symmetry-allowed in-plane TH generation pathways. Crystal orientation controls their nonlinear coupling, selectively enabling destructive interference for one incident helicity. Our nonlinear temporal coupled-mode theory identifies the conditions for near-unity CD, reveals the distinct roles of pump and TH resonances, and explains the fourfold and eightfold crystal-orientation dependence. Across silicon, germanium, and diamond, we obtain resonantly enhanced TH CD exceeding 99% in the near-infrared, establishing crystal orientation as a design degree of freedom for nonlinear CD in highly symmetric metasurfaces.