The present study sought to uncover the effect of previously acquired languages, Azeri and Persian, on the acquisition of English adjective properties of bilingual learners. It further investigated the role of L3 general proficiency and its interaction with the effect of previously acquired languages on the development of English adjective properties. More specifically, the aim of the study was to discern which of the theories in the field of third language acquisition (Cumulativeenhancement Model, Flynn & Foley, 2004; L2 Status Factor Hypothesis, Bardel & Falk, 2007; and Typological Primacy Model, Rothman, 2010, 2011, 2013, 2015) could account for the acquisition of adjective placement and order in L3 English. To fulfill so, 180 bilingual and monolingual university students from Miyandoab and Arak took a general English proficiency test, a background questionnaire, and a grammaticality judgment test. The data, then, were analyzed through descriptive statistics, multivariate ANOVA and post-hoc comparison tests. The obtained results indicated the effect of Persian L2 on the acquisition of English L3 adjective placement. Regarding adjective order, however, none of the theories could support the findings. Further investigation in this area will definitely lead to better theory building especially in the context of foreign language learning and instruction.