aaaPlural marking differs across languages. Some must mark plurality using an overt morpheme (e.g. English, Russian), while others mark it optionally (e.g. Korean) or lack an explicit plural morpheme (e.g. Chinese). This crosslinguistic difference in plural marking has received much attention in research exploring language transfer in the context of L2 acquisition, but little in the context of L3 acquisition. This study tests the effects of L1 and L2 knowledge on L3 acquisition with two groups of multilingual children (n = 68; 11-13 years old) with different L1s (Chinese or Russian) but the same L2 (Korean) and L3 (English). We also investigated whether L2 proficiency modulates influence of the previously learned language in L3 acquisition. The participants completed a timed acceptability judgment designed to investigate their knowledge of English plural marking. Results showed that the L1-Russian children were better able to distinguish between felicitous and infelicitous plural marking than the L1-Chinese children. In addition, the L1-Chinese children showed poorer performance as their L2 proficiency increased. The study discusses these results in light of theoretical approaches to L3 acquisition.