<Qua-2025-786>, hint

Suppose vertices A,B,C lie on the side of square K  and center of K be centroid G of ABC.

Then G is the center of symmetry and A', B', C' lie on the side of square. 

Let A', B', C' be symmetric to A ,B,C  respectively with respect to G.

If A lie inside K while B, C lie on side, then A' be inside K