Errors with "be", whether omission (e.g., "John happy") or overuse (i.e., "be"-insertion; e.g., "John is love Mary"), have received particular attention in L2 acquisition studies exploring L1 transfer. This study investigates such errors in the context of L3 acquisition, focusing on L1 transfer. L1-Chinese (n = 34) and L1-Russian (n = 34) children with L2 Korean completed an elicitation production task designed to explore their use of English "be." The study resulted in two main findings. First, L1- Russian children showed more omission errors than proficiency-matching L1- Chinese children, possibly due to an L1 transfer given that copula in Russian are dropped in the present tense. Second, L1-Chinese learners used "be"-insertion more frequently than proficiency-matching L1-Russian children, possibly due to using "be" for more functions (as a topic marker and an inflectional morpheme), as other research has shown for L2-English learners with topic-prominent L1s. Based on the findings, the study discusses some pedagogical implications.