In this work a mechanistic model is proposed to predict pressure drop and phase holdup for viscous oil-water-gas flows within horizontal duct. The model is suitable for three phase intermittent (slug) flows where oil and water phases are fully mixed. However, validation is also made for slug flow with core-annular oil and water flow. This approach makes use of a correlation for total slug unit length developed by Babakhani Dehkordi (2017) as an input in the model so that the continuity equation is explicitly solved, reducing complexity of the present models in the literature survey. Furthermore, oil and water are assumed to have a homogeneous behavior to simplify three-phase flow equations. The model predictions are compared with experimental data of viscous oil-water-gas slug flows. Results revealed that inclusion of slug unit data together with assumption of homogeneous flow for oil and water in mechanistic model improved prediction of pressure drop over the range of investigated flow conditions. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.