Grid forming inverters controlled as a virtual synchronous generator can improve the dynamic response of autonomous microgrids. However, the capability of operating in weak grid conditions and isolated mode of operation along with maintaining the voltage at the point of common coupling embodies multiple nonlinear dynamics which can be tackled by a control structure as a discontinuous state function known as sliding mode control. In this paper, a swing equation-based control architecture augmented with a nonlinear sliding mode control has been adopted to design a robust control for a three-phase grid-forming inverter. The proposed control structure can furnish non-oscillatory power to the load during unbalanced grid conditions. This system can operate in isolated mode and sustain the nonlinearities, which can arise due to changes in plant parameters. The performance of the proposed controller is verified through Hardware-in-the-Loop cosimulation results, which confirms the enhanced robustness of such a system.