This study aims at comparing the effects of order of acquisition (L1 vs. L2) and language of contact instruction on third language acquisition (L3), exploring whether the context of acquisition affects learners’ knowledge of the L3. To this end, four groups of L3 English learners were selected: The first and the second groups had Persian as their first language (L1) and Arabic as their second language (L2). The participants of the first group were students of Persian language and literature, while the participants of the second group were students of Arabic and literature. The third and fourth groups had Arabic as their first language and Persian as their second language. The participants of the third group were students of Persian language and literature, while the participants of the fourth group were students of Arabic language and literature. The Knowledge of English plural marking was elicited via a grammaticality judgment correction task and a picture description task, aimed at examining how these groups learn number agreement between the noun and its adjective modifier in English as their L3. Results revealed that Persian and Arabic A groups (those with Persian as their language of instruction) outperformed the other groups in both tasks, suggesting that they transferred plural marking facilitatively from Persian.