Mesh.uv3

Description

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

// Generate planar uv coordinates for all 4 uv sets

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.uv2 = uvs;
        mesh.uv3 = uvs;
        mesh.uv4 = uvs;
    }
}

Note: Texture coordinates have to be modified or created externally from the mesh. In the script example above the uvs is created as a new array. Then the texture coordinates are assigned to that array. Once these are assigned the array is assigned to the Mesh.