The purpose of this research is to explore the interaction patterns in an English (L2)-mediated Mandarin Chinese (L3) class (native-taught) for young monolingual Korean (L1) learners. Twenty-nine class videos were collected and transcribed into 152 pages of Chinese, English, and Korean for this analysis. The interaction patterns in the English-mediated Chinese classes were analyzed using the IRF framework of Sinclair and Coulthard (1975) and Tsui (1994). The results and discussions were integrated into four main patterns to interpret the English-mediated Chinese classes: 1) the native Chinese teacher’s needs for class assistance, 2) the students’ enthusiastic but inadequate use of English, 3) the students’ voluntary Korean scaffolding, and 4) the students’ class participation needs. On the one hand, this research has provided the students’ response to the teacher’s use of Chinese and English (in a foreign language-only environment), which is intended for a better understanding of a classroom with a trilingual teacher and monolingual students. On the other hand, the research has implications for understanding use of an L2 to facilitate an L3 classroom, particularly in the situation of using two foreign languages in one classroom.