This study sets out to investigate possible effects of previously learned languages on the acquisition of English as a third language by examining six prepositions (in, on, at, behind, over, to) when they denote spatial relations. Two picture description tasks were employed to find out which of the two known languages (L1 and/or L2) is the major source of cross-linguistic influence on the acquisition of English (L3) prepositions given the fact that adpositions have different morphosyntactic representations in Turkish, Kurdish and English. Turkish-Kurdish bilinguals learning English as a third language formed the experimental group and Turkish monolinguals learning English as a second language served as a control group in the study. The results revealed that the Turkish-Kurdish bilinguals have better performance in the comprehension and production of the target prepositions than the monolingual Turkish control group, particularly when there are structural overlaps (i.e. prepositions in both Kurdish and English) between the adpositional systems of L1-Kurdish and L3-English. The findings are suggestive of structural similarity and typological proximity as overriding factors in cross-linguistic influence in the acquisition of L3 English.