The operation and efficiency of 3-phase separators were investigated in this work using various simulation techniques. An overall model of a commercial 3-phase separation unit is established utilizing black-box modelling approaches and the open-source software DWSim. A gravity settling model based on the inlet conditions of the separator was created, which provides a general view of the water output quality of the separator in terms of oil residuals dispersed in the water outlet phase under dynamic conditions. Based on a test geometry of a typical separator in the oil and gas industry, this model was tested under specific working conditions that occur in oil production. The model shows a dynamic change of the oil residues at the water outlet of the separators at a start-up scenario in connection with the change in the water level. After reaching the optimum operating condition, the amount of oil in the water outlet settles at 14.21 kg/h. In the following simulation approach, a Computational Fluid Dynamics (CFD) simulation of the separator’s test geometry was created. The CFD model gives good results in the area of phase interaction between oil and water but needs further work to calculate the dispersion properties of the oil in water.