This paper presents a probabilistic approach to evaluate the summation results of the injected 3rd-order harmonic currents produced by several groups of micro hydro generator (MHG), where the MHGs are scheduled to be installed in several neighboring irrigational canals of Chia-Nan Plain of Southern Taiwan. Each MHG group has different MHG sets connected to an AC bus through the feeders of a distribution system through power-electronics converters consisting of several three-phase AC-to-DC converters, a regulated DC bus, and a large three-phase DC-to-AC inverter. A large harmonic-current source is also connected to the same AC bus to examine the effect of the external harmonic-current source on the summation of the 3rd-order harmonic currents. The mean value and the variance of Gaussian marginal probability density functions (pdf) and their covariance of the injected 3rd-order harmonic currents due to the switching of the power-semiconductor devices inside the power-electronics converters of the MHG sets are calculated and justified by Monte Carlo simulation. The simulation results show that the external large harmonic-current source can affect the probability distribution of the summation results of the injected 3rd-order harmonic currents generated by the MHG groups while the resultant probability distribution of summation of the injected 3rd-order harmonic currents is approximately normal.