Triangulation -- a combined use of different assessment methods or sources to evaluate psychological constructs -- is still a rarely used assessment approach in spite of its potential in overcoming inherent constraints of individual assessment methods. This paper uses field test data from a new OECD Study on Social and Emotional Skills to examine the triangulated assessment of 19 social and emotional skills of 10- and 15-year-old students across 11 cities and countries. This study assesses students' social and emotional skills combining three sources of information: students' self-reports and reports by parents and teachers. We examine convergent and divergent validities of the assessment scales and the analytical value of combining information from multiple informants. Findings show that students', parents' and teachers' reports on students' skills overlap to a substantial degree. In addition, a strong 'common rater' effect is identified for all three informants and seems to be reduced when we use the triangulation approach. Finally, triangulation provides skill estimates with stronger relations to various life outcomes compared with individual student, parent or teacher reports.