The concept of professional agency has recently been predominating in the field of teacher education. Despite this considerable attention given to exploring professional agency embedded in different contexts, there is still scarcity in research on teacher trainees' agentic roles. To this end, this study focuses on exploring the impact of practicum teaching on the construction of a professional agency of teacher trainees in three countries. Eleven teacher trainees (four from Turkey, three from Poland and four from Portugal) prepared reflection reports responding to 12 open-ended questions focusing on factors that contribute to or detract professional agency. The reflection reports displayed supporting and detracting factors about professional agency as self-related (responsibility, perseverance and sense of belonging to the profession), mentor-related (rapport, modelling, being a team member and decision-making, constructive feedback, allowing adequate space) and classroom and student-related issues (authenticity, role in students' eye, discipline and classroom management).