Please help me! Setting UVs on plane using World Space

Hello. I've made a C# script that generates triangle mesh with 3 Vector3 points.

and 3 points keep changing everytime the triangle mesh pops up.

 triangle = new GameObject ("triangle", typeof(MeshFilter), typeof(MeshRenderer));
   triangle.transform.position = new Vector3(0,0,-0.1f);
   vertices = new Vector3[]{pointA, pointB, pointC};
   triangles = new int[] {1,0,2};
   Vector2[] uv = {new Vector2(0,0)
       ,new Vector2(1,0)
       ,new Vector2(0,1)
       };
 
   mesh = new Mesh();
   mesh.vertices = vertices;
   mesh.triangles = triangles;
   mesh.uv = uv;
   meshFilter = triangle.GetComponent<MeshFilter>();
   meshRenderer = triangle.GetComponent<MeshRenderer>();
   meshRenderer.material = matTriangle;
   meshFilter.mesh = mesh;

I tried to put meterial on this triangle dynamically but the texture looks twisted.

how can I get a nice flat image like it's stuck in the background and masked with triangle? (like triangle shaped spot light)

the answers said that I should set UV in WorldSpace but I have no idea with WorldUV... :(

I've been searching for WorldUV for weeks but nothing was like my project.

someone please be my hero... thank you for reading by the way!