This detailed analysis of Swiss and German middle-school learners' negation and interrogation patterns, reveals significant differences in the way multilingual learners with varying ages of onset and distinct L1s, L2s and L3s produce their own rule systems when learning the application of periphrastic do in L3/L4 English. Their productive knowledge and grammaticality judgments cannot be fully accounted for by either their L1 structure (Swiss vs. Standard German) or by the target language (English), which points to L2 (French) incorporation in their L3 expressions and grammaticality judgments, thus backing the hypothesis of syntactic L2 to L3 transfer.