The purpose of this study was two-fold: it primarily intended to incorporate some insights into the newly-explored field of L3A: secondly, it aimed to highlight the significance of translation as a valid language activity in exploring the native language influence on non-native language acquisition process. To this end, it investigated the acquisition of two syntactic properties of head and operator movements in English by L2 and L3 learners within UG framework. The participants consisted of 144 Persian monolingual and Arabic-Persian bilingual learners of English who were assigned to three proficiency bands after taking the general proficiency test (ECPE). The results showed no significant difference between the performance of monolinguals and bilinguals at each level of proficiency. Nonetheless, significant differences were found across the levels of proficiency.