We develop a grain-based model for capillarity controlled displacement within 3D fractionally wet porous media. The model is based on a novel local calculation of the position of stable fluid-fluid interfaces in contact with multiple spherical grains of arbitrary contact angles. The interface is assumed to be locally spherical between bulk phases; the interface is assumed to be toroidal between pairs of grains (surfaces of pendular rings). Because the calculation of interface position is entirely local and grain-based, it provides a single, generalized, geometric basis for computing pore-filling events during drainage as well as imbibition with both Melrose events (merging of two interfaces) and Haines events (geometric instability). The model is validated against a series of drainage/imbibition experiments (oil/water) on fractionally wet porous media prepared by mixing oil-wet grains with water-wet grains.