In an era of globalization, multilingualism is no longer the exception but the rule. To fully promote the communicative and functional development of multilingual learners, different educational programs at school have been implemented in the past few years. The study presented here is a contribution in this direction, by investigating the linguistic development in a second foreign language (L3 English) of students who were exposed to asymmetric input exposure in this and another non-native language. This other language was the L2 (German), which the learners (n= 50, 11-13 years old) were studying in a CLIL context together with the native languages of the participants (Spanish and Catalan). Within such a context, the purpose of the study is two-fold. On the one hand, it attempts to investigate the linguistic attainment in the L3, measured in terms of various dimensions of writing performance in a composition task, namely, fluency, accuracy, structural complexity and lexical complexity. On the other hand, the study aimed at exploring the contribution of various components of working memory (short-term memory and attention switching) to performance in these linguistic dimensions. The results suggest that individual differences in storage capacity and in capability for attention and task switching had an effect on the accuracy of the learners' performance, and to a lesser extent, also lexical complexity. Moreover, it was found that in general, attentional abilities had a greater impact than memory. These results are discussed in light of previous findings, while proposing directions for further investigation in the field.