The paper addresses concept formation processes of students in the field of geometry. More precisely, the paper deals with the questions of what knowledge do students activate about triangles in different contexts with different (digital) tools and furthermore what content-related meaning do they give to the concept of triangles? Methodologically, we use the descriptive framework of empirical theories for the analysis of our case study. The students develop different notion of the concept of triangles, using three empirical contexts like 3D pen, dynamic geometry software and pencil paper in an activity that allow students to construct the characteristics of triangles. In this article, we focus on how the students in our case study (further) develop and give meaning to the concept of triangle, which is central in geometry, in three empirical contexts; thus, engage in concept formation processes.