This paper presents a precise description of the fluid distribution and pore-scale displacement mechanisms for three-phase flow under strongly wetting conditions when the displacing fluid is a nonwetting phase. It is shown that on the pore-scale the fluids may adopt one of three basic configurations depending on the values of the three interfacial tensions and the wetting preference of the solid. The nature of the three-phase displacement mechanisms is determined by the pore-scale fluid distribution. The displacing phase may advance by two basic mechanisms; a double drainage mechanism involving all three phases - a three-phase displacement - or, a direct drainage mechanism - a two-phase displacement. The three-phase displacement mechanism is described by a simple generalisation of two-phase flow mechanisms. The basic displacement mechanisms are incorporated into a numerical percolation-type network model which is used to compute phase recoveries for three-phase displacements. Computed recoveries are shown to be in good agreement with those determined experimentally. The model may therefore provide a basis for modelling three-phase Rows in actual porous media.