The Three Field Theory (3FT) Formulary v1.8 is the 2026 reference edition of the Arc-Time Evolution (ATE) framework. It presents the geometry, tension, and symmetry fields under a discrete tick-based update scheme and unified action, together with the lattice architecture, primitive set {ℓ₀, κₛ, χ_ℋ}, core laws, calibration protocols, falsifiability gates, and reproducibility appendices. This edition is a working release intended as a stable reference baseline for audit, comparison, and external review. It is not a claim of theoretical closure. The document fixes v1.8 scope, authority hierarchy, deterministic seed = 42, and audit reference SHA beb2c2b7.