A graph structure F (Σ) can be given to the triangulations of a surface Σ with a prescribed set of vertices. Its edges connect two triangulations that differ by a single arc. An interesting question is whether the subgraph Fε(Σ) induced in F (Σ) by the triangulations that contain a given arc ε is strongly convex in the sense that all the geodesic paths between two such triangulations remain in that subgraph. A positive answer to this question has been given when Σ is a convex polygon or a topological surface. Here, we provide a related result that involves a triangle instead of an arc, in the case when Σ is a convex polygon. We show that, when the three edges of a triangle τ appear in (possibly distinct) triangulations along a geodesic path, τ must belong to a triangulation in that path. We also provide two consequences of this result. The first consequence is that Fε(Σ) is not always strongly convex when Σ has either two flat vertices or two punctures. The second is that the number of arc crossings between two triangulations of a convex polygon Σ does not allow to approximate their distance in F (Σ) by a factor of less than 1.25.