The flow of three fluid phases – typically oil, water and gas – is described from a pore-scale perspective. Spreading and spreading coefficient are first described, followed by a description of wettability and the derivation of the Bartell-Osterhof equation which presents a constraint between the three contact angles. The pore-scale arrangement of fluids is described with an analysis of oil layers and consequences for oil relative permeability at low saturation. Then displacement processes are introduced with two new phenomena seen in three-phase flow, namely multiple displacement and self-consistency. Relative permeabilities in three-phase flow are presented based on measurements in the literature and compared, where available, with predictions from network modelling. The saturation dependence of relative permeability and capillary pressure is explained in terms of pore occupancy. The pore-scale behaviour is used to explain trends in relative permeability with wettability and spreading coefficient. Trapping is discussed and experimental results are interpreted in terms of pore-scale displacement processes. Direct images of trapped phases are shown, with evidence for the existence of oil layers. Last, empirical models of three-phase relative permeability are briefly described and critiqued.