Second language studies on Korean language have noticeably increased over the last decade due to the development of Korean education in the U.S. Despite the fact that the majority of Korean classes are enrolled in by Korean heritage learners, the growth of non-heritage learners is also a noticeable trend. Most of the studies available have focused on the acquisition of either Korean heritage learners or on the acquisition of Korean as a second language. Apart from earlier studies, the purpose of this study is to examine prior language knowledge influence in the case of English L1 speakers with Japanese L2 when acquiring Korean as L3. The study aims to investigate the cross-linguistic influence on L3 Korean language acquisition, particularly for case and post-position production in intermediate level. Two types of samples were collected for this study. The experimental groups were consist of L3 learners of Korean whose L1 is English with Japanese L2, and the control group was L2 learners of Korean whose L1 is English. Samples in the experimental groups were tested both for Korean and Japanese post positions whereas the control group was tested only for the Korean particles. The statistical results demonstrate significant divergence among groups. The results of the experimental groups manifest both positive and negative transfer indicating an interface between L2 Japanese and L3 Korean.