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Important Update on March 9, 2026: In this version, the description of environmental effects has been corrected: they are now properly treated as encapsulated within the effective spectral density of the interfacial defect network D, rather than as direct driving terms. All core results and conclusions remain unchanged, but the physical interpretation is refined to align with the energy distribution framework introduced in Part II of this series. Version 2 This paper is the third in the "Interface-Projected Realism" series. This series of work aims to establish a new theoretical paradigm: Ontologically, replace probabilistic wave function descriptions with deterministic energy flow; Methodologically, redefine physical reality as an interactive whole composed of quantum units ($Q$), an interfacial defect network ($D$), and bulk channels ($B$). In the first work, an exact relationship between energy flow and relaxation rate was established, providing a formal bridge for the paradigm shift. In the second work, the "Energy Distribution Framework" was proposed, demonstrating that dissipation is the deterministic allocation of energy between $Q$ and $D$, and deriving the universal formula for internal dissipation power $R_{\mathrm{int}} = \hbar\omega_q[\Gamma_\downarrow\langle n\rangle - \Gamma_\uparrow(1-\langle n\rangle)]$, where the relaxation rates are completely determined by the internal structure of $D$, revealing the core role of the energy distribution ratio $f$. This paper generalizes this new paradigm based on energy distribution from single-body to many-body systems. The core step is to define multiple $Q$ contacts and their shared $D$ and $B$ together as a new, unified composite system. Within this framework, the traditionally complex "many-body correlated dissipation" is clearly interpreted as a macroscopic result of the deterministic flow of energy within this composite system, governed by allocation and transport rules determined by the structure of the $D$ network. Through rigorous coarse-graining of this overall system, this paper derives the governing equation for the evolution of its internal energy density field $u(\mathbf{r},t)$—a classical reaction-diffusion equation:\[\frac{\partial u}{\partial t} = R_{\mathrm{total}} - \gamma_0(\mathbf{r}) u + \nabla \cdot (\gamma_2 \nabla u).\]Here, the local dissipation rate $\gamma_0(\mathbf{r})$ and the diffusion coefficient $\gamma_2$ are completely determined by the properties of the $D$ network, corresponding respectively to the spatial distribution of the energy distribution ratio $f$ from the second paper and the redistribution effect induced by its correlations. This theory is not only logically self-consistent but also provides a unified explanation for multiple cutting-edge experiments. This paper marks that a physical framework based on deterministic energy distribution and transport can now systematically describe many-body correlation phenomena, providing crucial support for the completeness of the new paradigm. \textbf{Keywords}: Interface-Projected Realism; Energy Distribution; Many-Body Systems; Correlated Dissipation; Reaction-Diffusion Equation Version 1 Traditional quantum theory typically attributes energy dissipation to random perturbations from the external environment. Based on the "Interfacial Projected Realism" framework, this work re-examines the nature of energy dissipation from a novel perspective, treating the qubit, the interfacial defect network, and the bulk dissipation channel as a self-contained total system. We first rigorously derive the universal formula for internal dissipation power and the energy distribution ratio \(f\) in single-body quantum systems, and then extend these concepts to many-body systems. Starting from a microscopic Hamiltonian coupling multiple qubits to a common quantized field and employing the Nakajima-Zwanzig projection operator technique, we obtain a universal expression for the energy dissipated from a many-body system to the environment. Applying this result to the specific model of multiple two-level systems coupled to a common bosonic field yields a collective master equation where the dissipation coefficient matrix \(\Gamma_{ij}\) encodes all spatial and coupling information. Through systematic coarse-graining, gradient expansion, and semiclassical approximation, we derive for the first time from an energy-distribution perspective a closed set of macroscopic equations for the local system energy density \(u(\mathbf{r},t)\) and the environmental field energy density \(U(\mathbf{r},t)\): \[\frac{\partial u}{\partial t} = R_{\mathrm{total}}(\mathbf{r},t) - \gamma_0(U) u + \nabla \cdot \bigl( \gamma_2(U) \nabla u \bigr)\]\[\frac{\partial U}{\partial t} = D \nabla^2 U - \Gamma_B U + \rho(\mathbf{r}) \gamma_0(U) u\] Within this framework, the energy distribution ratio \(f(\mathbf{r},t) = \gamma_0 u / R_{\mathrm{total}}\) emerges naturally as a macroscopic quantity, its physical essence being jointly determined by the system’s microscopic coupling structure (embodied by \(\Gamma_{ij}\)) and quantum statistical state. The theory quantitatively explains several long-unexplained anomalous phenomena in cutting-edge experiments, demonstrates strong testability, and lays a solid foundation for exploring deeper connections between energy distribution and macroscopic physics. For the exact mathematical formulation and detailed derivations, please refer to the PDF file.