Is it a problem if we make 3 triangles that has 1 vertices in common like in this picture?
It depends. Look at it in Object Mode and see if it gives Shading artifacts. I think in your case it is probably not okay (as it is now),
but on a flat surface, away from the Edges it can be totally fine:
thx!
I would avoid those kinds of terminations on a vertex that is on the edge as holding edges. That most probably will create some pinching.
