This preprint is published as part of the author’s theoretical series, “Fundamental Being Space (FBS) Theory.” In this series, we introduce a working hypothesis in which the interface layer Σ and the Fundamental Being Space (FBS) are postulated in addition to ordinary spacetime S_i. By examining this framework, we show that a consistent description can be formulated that connects with established physical theories, and we present the results in a step-by-step manner. Overview of Paper 3This paper investigates why gravity is extremely weak compared to electromagnetism. Rather than treating the hierarchy as an arbitrary parameter choice, we model the interface layer Σ as an effective “filter” between FBS and ordinary spacetime S_i. We introduce a multi-sublayer projection/transmission picture in which attenuation accumulates across sublayers, allowing the effective gravitational coupling to be exponentially suppressed while remaining consistent with key experimental constraints, including inverse-square-law tests, equivalence-principle tests, and gravitational-wave observations. We also position the proposed mechanism relative to representative viewpoints on gravity (general relativity, precision tests, and emergent-gravity perspectives), and state clear conditions under which the filtering mechanism would be falsified. Electromagnetism (Paper2) provides the geometric projection mechanism (double-helix → A → E,B) used throughout the series. Series notePaper 3 of the “Fundamental Being Space (FBS) Theory” series. Recommended citationYoshihito Ookawa, Geometric Origin of Gravitational Weakness from Interface-Layer Filtering, Zenodo, 2026. (DOI: 10.5281/zenodo.18240764) Series navigation Paper 1 (Overview / Series map)General discussion of the “Fundamental Being Space (FBS) Theory” framework and the series-wide mapping.Zenodo: https://zenodo.org/records/18220715 Paper 2 (Electromagnetism / Double-helix → A → E,B)Derives an effective vector potential A from the geometry and phase difference of a double-helix structure in the interface layer Σ, recovering electromagnetism (E, B) and the Maxwell equations in the long-wavelength limit.Zenodo: https://zenodo.org/records/18232306 Paper 4 (Tachyonic modes / Light-speed hierarchy Ci)Reinterprets “tachyon” as a mode (not a particle) and discusses stabilization of the light-speed hierarchy Ci with consistency to dispersion relations.Zenodo: https://zenodo.org/records/18327253 Paper 5 (Time generation / Coupled constraint ΔC–ΔT)Formulates time generation as a constraint/coherence phenomenon in Σ via a coupled condition between the velocity width ΔC and the time step ΔT.Zenodo: https://zenodo.org/records/18327480 Recommended path: Paper 1 → Paper 2 → Paper 3 → Paper 4 → Paper 5 VersioningThis preprint may be updated in revised versions. When citing, please specify the DOI and the version. Author informationIndependent Researcher, JapanORCID: 0009-0006-8294-5738