Mesh.uv3

Description

The third texture coordinate set of the mesh, if present.

// Generate planar uv coordinates for the third uv set

using UnityEngine;

public class ExampleClass : MonoBehaviour
{
    void Start()
    {
        Mesh mesh = GetComponent<MeshFilter>().mesh;
        Vector3[] vertices = mesh.vertices;
        Vector2[] uvs = new Vector2[vertices.Length];

        for (int i = 0; i < uvs.Length; i++)
            uvs[i] = new Vector2(vertices[i].x, vertices[i].z);

        mesh.uv3 = uvs;
    }
}

Note: Texture coordinates must be created or modified externally from the mesh. The script example above creates the uvs array, assigns texture coordinates to the uvs array, and then assigns the array to the Mesh.