In Inner Mongolia Autonomous Region, many Mongolian students are learning English as a third language. In the process of L3 teaching and learning, their mother tongue Mongolian, second language Chinese and target language English are involved. The present paper aims to find out teachers’ and students’ opinions of the use of the three languages in the third language teaching and learning by Mongolian students at university and the reasons to this phenomenon are discussed. Furthermore, their English learning experience and L2 proficiency are explored. The main findings revealed that Chinese teachers believe that English should be used to the fullest to maintain the maximum use of the target language with the help of Mongolian for the support; however, the majority of Mongolian teachers and Mongolian students believe that Chinese can be used as the main medium instructional language for various reasons, and it is effective to use all the three languages according to the different stages of teaching and the proficiency levels of the three languages.