You want to get two normals on the ‘plane’ of the vertex, the take the cross product to get the normal of that plane (which is the normal of that vertex).
I’m not sure what data you’re using to drive this mesh right now, but you want to sample it at 4 points around your point, make two vectors out of those 4 points and cross them.
In that particular quad’s case:
FVector First = Triangles[1] - Triangles[0];
FVector Second = Triangles[2] - Triangles[0];
Normals[0] = FVector.CrossProduct( First, Second ).Normalized();