The virtual poster sessions of the 3rd Summit on Gender Equality in Computing (GEC 2021) offer the opportunity to undergraduate, graduate and PhD students, as well as young researchers and professionals of any gender, to share insights and knowledge with other participants, senior researchers and professionals in the field, to disseminate their research work, to practice their presentation skills, and to build confidence in presenting research work in a succinct manner. The poster sessions include a total of 27 posters. All poster submissions were peer reviewed by the Poster Evaluation Committee and all accepted poster presentations are made available here, on the Greek ACM-W GEC 2021 website.