In this study, we investigated crosslinguistic influence (CLI) at developmental stages of third language (L3) acquisition of English by Russian--Norwegian children (N = 31). We tested seven linguistic properties within three linguistic modules (morphology, syntax and syntax-semantics). We compared the L3 learners to Norwegian (N = 90) and Russian (N = 74) second language (L2) learners of English. We predicted simultaneous facilitative and non-facilitative CLI in the L3 group within all modules, as the previously acquired languages offered conflicting options. Our predictions were partly supported. On one property, the L3 learners were different from both L2 groups, which is in line with cumulative CLI from both previously acquired languages. On four conditions, the L3 learners performed like the more accurate L2 group, indicating facilitative influence. On two conditions, all groups performed alike, showing high rates of accuracy. Taken together, the results indicate that CLI obtains on a property-by-property basis, with none of the L1s being the sole or primary source of CLI. Finally, we found CLI in all linguistic domains, but the developmental slopes for the properties were not equal, which suggests that factors such as complexity and saliency needs to be taken into account when we compare CLI.