The study explores the impact of language attitudes on third language (English) writing. A total of 40 participants (20 from the B2 level and another 20 from the C2 level) from an Official Language School in the Valencian Community took part in the study. Data were collected by means of semi-structured interviews and a questionnaire adapted from Lasagabaster and Huguet (2007), together with and a written composition evaluated using Jacobs et al.’s (1981) taxonomy. Findings from the study show that students' attitudes towards languages are in line with the sociolinguistic status of each language in the Valencian Community. In addition, the present study shows that those students that use both official languages (Spanish and Catalan) at home got better results in the evaluation of their English writings. Our findings indicate the impact of multilingualism on students' L3 writing and suggest the advantages of multilingualism in language learning.