In this article, we compare holistic, analytic and cross-linguistic measures of assessment in writing considering the whole linguistic repertoire of multilingual writers. We explore different ways to look at their three languages by focusing on the relationships between these languages rather than looking at each language in isolation. We analyse 399 compositions -- 133 written in each language (Basque, Spanish and English) -- using quantitative and qualitative methods. Using analytic accuracy measures we compare the error-types in the three languages and analyse their relationships. Using holistic measures, we focus on the relationships between five dimensions of writing (content, organization, vocabulary, language use and mechanics). Using cross-linguistic measures, we analyse the influence of participants' first and second language on the compositions written in English (L3). The results of the correlation analyses show different patterns of relationships among the measures and indicate that languages within a person's repertoire are interconnected at different levels. Additionally, the results from the cross-linguistic analysis show that multilingual writers use their entire linguistic repertoire to write in their third language. We argue that analysing the entire linguistic repertoire of multilingual writers we gain new understanding about the way languages are connected and learned when writing.