Mesh generation: One mesh is visible only in 1 view

Hi,
I generated 3 meshes, but one of the 3 is not visible (triangle 0 1 3).

EDITED:
OK, after trying different disposition of vertices in _Vertices2, it seems I got it working (the green triangle in attachment) - but I need an explanation of why it is working. Can someone please advice?

Vector3[] __vertices = new Vector3[] { _vertices[2], _vertices[1], _vertices[0] };
int[] __tris = new int[] {0, 1, 2};
Vector3[] __normals = new Vector3[] { -Vector3.forward, -Vector3.forward, -Vector3.forward };
GenerateMesh(__vertices, __tris, __normals);

Vector3[] __vertices1 = new Vector3[] { _vertices[2], _vertices[0], _vertices[3] };
int[] __tris1 = new int[] {0, 1, 2};
Vector3[] __normals1 = new Vector3[] { -Vector3.forward, -Vector3.forward, -Vector3.forward };
GenerateMesh(__vertices1, __tris1, __normals1);

Vector3[] __vertices2 = new Vector3[] { _vertices[0], _vertices[1], _vertices[3] };
int[] __tris2 = new int[] { 0, 1, 2 };
Vector3[] __normals2 = new Vector3[] { -Vector3.forward, -Vector3.forward, -Vector3.forward };
GenerateMesh(__vertices2, __tris2, __normals2);

P.S. I need to generate one triangle per time (that's why I invoke Generate mesh 3 times instead of 1)

 

Could it be vertex winding order? Unity assumes that vertices are always specified in clockwise order, so if you specify the vertices in counter-clockwise order (relative to the camera) then your triangle will be facing away from the camera, making it invisible.

 

Yeah that seems to be it. Thanks!