There is quite a bit of evidence showing that the experience of learning an L2 will help in learning an L3, but as far as we know, very little research has investigated the possible impact of L3 learning on the already existing and still developing L2 system within the learner. According to Complex Dynamic Systems Theory (CDST), language development depends on limited resources. In simultaneous L2 and L3 (L2 + L3) learners, these resources have to be used for learning two languages, reducing the resources available for L2 learning. This might lead, firstly, to a slower L2 development in L2 + L3 learners than in L2 only learners, and secondly, to more variability in the L2 during the learning process. In the current study, we traced the L2 English writing proficiency scores (both in terms of gains and variability) of two groups of L1 Chinese learners within one academic year. One group learnt English only (L2) and the other group learnt English and Russian simultaneously. Results show that the L2 + L3 learners did not develop their L2 to a lesser extent than the L2 learners did, but they showed more variability over time in one sub-area (fluency) of L2 writing proficiency. The implications are discussed.