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All objects that burn in fires are three dimensional. However, almost all previous attempts to model the burning or pyrolysis of real materials has resorted to a one-dimensional description. This paper presents the mathematical formulation of Gpyro3D, a generalized three dimensional pyrolysis model that solves conservation equations for transport of heat, mass, and species in a chemically reacting porous medium, as well as its coupling to NIST’s Fire Dynamics Simulator (FDS) for simulating fire development. Gpyro3D facilitates pyrolysis modeling in complex geometries by masking grid cells on a regular Cartesian grid, similar to the way that geometry is specified in FDS. The model’s mathematical formulation is verified by comparing numerical simulations to analogous exact solutions. Next, it is shown that Gpyro3D qualitatively captures the major three dimensional features of long-duration (> 1 hour) oxidative pyrolysis of wet wood under radiative heating. Finally, fire development in a wood crib is simulated with the coupled Gpyro3D/FDS model. By observing condensed-phase temperature contours within burning elements of the wood crib, it is shown that strong three dimensional effects are present.