In this paper, a methodology to generate combined wave, tidal current and turbulent flow environment for application to determining hydrodynamic loading on a horizontal axis Tidal Stream Turbine (TST) is proposed. A systematic investigation has been conducted within the framework of OpenFAST software suite. Necessary modifications in OpenFAST tool is made to simulate the hydrodynamics of the TST under combined wave-current-turbulence conditions. The modified software will be able to generate the inflow conditions for the TST accounting for the regular and irregular wave and current interactions, and, involve the impact of ambient turbulence on the loadings of TST. The modified algorithm is validated by comparing the numerical results with experimental test data and other numerical simulation results. Good agreements are achieved for both regular and irregular wave kinematics in time-domain and flow velocity/elevation spectra in the frequency domain. Also, each of the wave-current configuration has been tested with different turbulence intensities. The results suggests that the wave kinematics is greatly altered when waves propagating on currents and wave travelling on the following and opposing directions current shows remarkably different characteristics in relation to wave amplitude and frequency. It is also found that combined wave-current flow with different turbulence levels have strong correlations with the flow properties which is important for TST fatigue prediction. All the conclusions emphasis the fact that proper wave-current-turbulence interactions should be included in TST modelling.