Second language (L2) writing researchers have demonstrated that bilingual writers draw on their native language to fulfill task requirements. A growing number of individuals are learning a language beyond their second, and, despite an increased interest in third/additional language (L3/Ln) development, few empirical studies have considered writing practices from a plurilingual lens. To fill this gap, nine multilingual writers (Spanish/French/English) completed two individual writing tasks (argumentative essays), one in French and one in English. The stimulated-recall interview data confirms that writing is a plurilingual activity. The participants drew on their cumulative linguistic repertoire to generate ideas, to structure their essays, and to reflect on language; however, for lexical reflections, their additional knowledge, rather than facilitate lexical accuracy, appeared to interfere with their writing experience. The findings, discussed from a plurilingual lens, confirm that individuals have fluid and partial competences of various languages which support them during academic writing tasks. Implications for cross-linguistic pedagogy are discussed.