Objective: With the continuous development of power grids, overhead transmission lines inevitably pass through regions with complex terrain and climatic conditions. In extremely cold environments, ice accumulation occurs on these lines. Under wind loads, the accumulated ice layers vibrate and detach, potentially causing accidents such as broken lines and tower collapses. The DC deicers serve as the primary equipment for power networks to defend against freezing disasters. They utilize Joule heating to raise line temperatures and melt ice and snow. However, existing DC deicers suffer from large size, heavy weight, and difficult transportation. As a core component of DC deicers, the rectifier transformer accounts for more than 70% of the total weight. To improve the mobility of these devices, it is necessary to optimize the structure of the rectifier transformer and reduce its weight. Methods: Based on the transformer theory, a three-phase three-dimensional (3D) symmetric core and its lamination method for rectifier transformers are proposed. The electromagnetic field equations and the phasor relations of main and mutual fluxes were derived according to Ampere's law. A finite element model of a 4 MV·A rectifier transformer was established in Ansys Electronics Desktop. The time-varying law of the magnetic field and the characteristics of the loss density distribution were analyzed. The limitations of existing numerical calculation methods in dealing with the microscopic behavior of magnetic domains and the additional loss in the vertical symmetry plane were analyzed. To verify the analysis results, a 4 MV·A prototype was fabricated and tested under thermal cycling and no-load conditions. Finally, an additional loss shape function expression with the average flux density gradient as the independent variable was proposed. The undetermined coefficients and loss correction formula were obtained by collecting no-load test data of samples with different capacities. Results: The mutual fluxes of the three-phase 3D symmetric core were separated by the vertical symmetry plane of each phase. The core flux density was $\sqrt{3}$/2 of the mutual flux density at any instant. The no-load loss of the 4 MV·A rectifier transformer (calculated using the finite element method) was 4.46 kW, with a maximum value of 1.2×10⁴ W/m³. The loss inside the core was substantially higher than that outside. Considering the microscopic motion of magnetic domains, there theoretically existed an additional loss near the vertical symmetry plane that could not be elucidated by existing algorithms. In the no-load and thermal cycling tests, the measured core loss was 9.73 kW, with an error of 118.16% compared with the calculated value. An obvious temperature rise was observed near the vertical symmetry plane of each phase core, verifying the existence of the proposed additional loss. Three-phase 3D symmetric core transformers with different capacities had similar shape functions, and the coefficients of the proposed correction model were approximately linear with the capacity. Conclusions: By analyzing the electromagnetic characteristics of the proposed three-phase 3D symmetric core, its core loss characteristics and a correction formula were obtained. This study can provide theoretical support for the engineering application of 3D symmetric core rectifier transformers and thereby contribute to the lightweight design of DC deicers. This, in turn, can improve the transportation capability of DC deicers and ensure the winter safety of power grids.