Teacher training programmes have often ignored the new multilingual paradigm and, consequently, educators may hold some misconceptions about how additional languages are learnt and should be taught in multilingual contexts. A number of recent studies have investigated language teachers’ beliefs regarding multilingual education (Arocena-Egaña, Cenoz & Gorter, 2015; De Angelis, 2011; Griva & Chostelidou, 2012; Haukås, 2016; Mitits, 2018; Otwinowska, 2014). Unlike this extant literature, the current paper focuses not on language teachers, but on a sample of 121 subject teachers-to-be. More specifically, it aims to explore preschool and primary prospective teachers’ beliefs about the learning and teaching of English as an L3 in the Valencian Community. The main data collection tool was a questionnaire, partially based on Griva and Chostelidou's (2012) semi-structured interviews. This questionnaire was implemented as a pretest and as a posttest in order to analyse the effect of instruction for multilingual education on subject teachers’ beliefs. Results point to the effectiveness of initial teacher training on tertiary/additional language teaching. However, they also confirm the persistence of some monolingual views concerning key issues, such as the younger-the better myth, as already highlighted in previous studies conducted in similar multilingual contexts (see Arocena-Egaña, Cenoz, & Gorter, 2015).