Introduction: Standard ECGI methods estimate epicardial potentials, which limits their applicability in clinically relevant scenarios, particularly when arrhythmogenic substrates are located intramurally or endocardially. Methodology: We propose a novel ECGI methodology for estimating volumetric cardiac sources. Using Green's functions, the approach establishes a direct and physiologically consistent link between internal sources and body surface measurements. To assess its performance, the method is compared to classical epicardial ECGI in three simulations of premature ventricular ectopic beats, evaluating local activation time maps. Its clinical applicability is further explored through tests on three patients with distinct arrhythmic conditions. Results: The Euclidean distance between the estimated and simulated origins of activation is reduced with the volumetric method from 25.50mm ±3.96mm to 12.26mm ±0.79mm. Additionally, epicardial ECGI shows a high amplitude ratio (51 : 1) between epicardium and endocardium, indicating poor depth accuracy. Volumetric ECGI shows a lower ratio (1.64 : 1), suggesting a more physiologically reconstruction across the myocardial wall. Patient results show strong concordance with electroanatomical maps and 12-lead ECGs, supporting the method's potential for clinical use. Conclusions: These results indicate that the proposed approach enables the estimation of 3D cardiac sources in both simulated and clinical scenarios.