The OpenFOAM Fuel BEhavior Analysis Tool OFFBEAT is a multi-dimensional fuel performance code under development at the École Polytechnique Fédérale de Lausanne (EPFL) and the Paul Scherrer Institute (PSI), in Switzerland. The latest developments are oriented toward the enhancement of the code's multi-dimensional capabilities via the substitution of traditional models inherited from legacy codes with more mechanistic approaches. As part of these efforts, the neutronics model currently used in OFFBEAT has been identified as a potential source of inaccuracy for multi-dimensional analyses involving asymmetric geometries or irradiation conditions. This simplified model derived from the TURBNP module of TRANSURANUS makes use of pre-calculated radial neutron flux distributions obtained from the solution of neutron diffusion equation under the assumption of axisymmetric irradiation and perfectly cylindrical conditions. If applied to 2D/3D geometries characterized by asymmetries such as a missing pellet surface defect or by localized details such as pellet dishes and chamfers, these pre-computed radial flux distributions might impact the fuel power conservation and profile. To allow for an extended applicability of the neutronics solvers in fuel behavior analysis, a new 3D diffusion-based neutronics solver is implemented in OFFBEAT. The proposed approach consists in solving the one-group neutron diffusion equations within the fuel pin domain, providing a reliable tool for the prediction of the 3D neutron flux distribution for any type of pellet geometry and irradiation conditions. This paper presents the details of the implementation of the solver in OFFBEAT, together with its comparison against solutions provided by the Monte Carlo code SERPENT.