An essential task in smart grids is the monitoring of power grid operation - especially, if grids are small (microgrids) and energy generation is volatile. Since simulations cannot cover all aspects of device operation, it is crucial to test monitoring devices in the real world. For this purpose, we develop an open-source and low-cost arbitrary 3-phase function generator that we describe in the present contribution. Its graph-based graphical user interface eases the programming of complex combinations of stationary, transient, frequency modulated, and phase-imbalanced signals. The hardware comprises a single-board computer (SBC), a DC-enabled digital-to-analog converter (DAC) (using components from the audio domain), and a plus/minus 10 V interface amplifier. The paper also presents the results of measurements taken in order to verify the generator's performance. In an application example, we demonstrate the use of this generator to test energy monitors in a high distortion scenario. Finally, a link to the open source repository is given.