The modular multilevel converter (MMC) has become one of the most promising topologies for high- and medium-voltage applications. The steady-state analysis of MMCs provides guidance for system design and performance evaluation. The existing steady-state analysis models developed for three-phase three-wire MMC systems can only deal with balanced ac-side circuits and symmetric arm conditions. And they fail to provide accurate analysis considering unbalanced ac-side circuits or when the components' parameters in different arms of the MMC are different. To address this problem, a generic steady-state analysis model is presented in this paper to precisely characterize the steady-state harmonic performance of the three-phase three-wire MMC under various operation conditions. An industrial-level MMC simulation system is built to demonstrate the feasibility and validity of the proposed model.