The question of why there are exactly three quark generations has no answer in the Standard Model — it is an empirical input, not a theoretical output. We show that in the One-Octonion Brane-Bulk Framework it is a mathematical theorem. The Fano lattice is naturally identified with the Klein quartic: the unique genus-3 Riemann surface with automorphism group PSL(2,7) of order 168, the maximum for genus 3 by the Hurwitz theorem. Yang (2024), following Ratcliffe and Thurston, proves (Lemma 3.4) that any compact hyperbolic surface of genus g ≥ 2 decomposes into exactly 2g−2 pairs of pants using exactly 3g−3 cutting geodesics, with each pair of pants uniquely determined by its three boundary lengths (Proposition 6.5, proved via right-angled hexagon uniqueness). For g=3: exactly 4 pairs of pants, exactly 6 cutting geodesics. The framework identifies the 4 trinions as: 3 quark generation sectors + 1 lepton sector. The 6 cutting geodesics form a complete graph K₄ connecting all 4 trinions pairwise, giving 6 inter-sector boundary geodesics corresponding to the 3 CKM mixing angles (quark– quark) and 3 PMNS mixing angles (quark–lepton). Since the {7,3} tiling fixes all 6 boundary lengths and G₂ fixes all 6 twist angles, Yang Theorem 8.3 (T(M) ^{6g−6} = ^{12}) shows the physical Fano ≅ℝ ℝ surface is a unique point in Teichmüller space — zero free parameters. The number 3 is not a coincidence, not a mystery, and not an input: it is the genus of the Klein quartic minus one, forced by the 7-node structure of the Fano plane through the Hurwitz theorem.Part of the One-Octonion Brane-Bulk Framework series. Anchor DOI: 10.5281/zenodo.19120873. Community: one-octonion-brane-bulk. Author: Bharathi Dasan Jagadeesan, M.D., University of Minnesota. ORCID: 0000-0002-1143-941X.