The present study tested whether the switch costs of production-based language switching tasks by trilinguals are due to competition from outside language schemas or from within language systems. We recruited 90 Tibetan-Chinese-English trilinguals in 10th grade in a Tibetan middle school in China to name digits and number words using different stimuli valence. The study was divided into three experiments, one for each combination of two languages possible amongst Tibetan, Chinese and English. We observed the switch cost symmetrical patterns in the three experiments. The results suggest that the sources of switch cost are partially from outside competition between language schemas and partially from within language system. These results may contribute to our understanding of the language control mechanism between production-based and comprehension-based language switching tasks and language control mechanism of trilinguals with three orthographically different languages.