The index of the triangle that was hit.

	// Attach this script to a camera and it will
	// draw a debug line triangle triangle
	// at the triangle where you place the mouse.
	function Update () {
		// Only if we hit something, do we continue
		var hit : RaycastHit;
		if (!Physics.Raycast (camera.ScreenPointToRay(Input.mousePosition), hit))
			return;
		// Just in case, also make sure the collider also has a renderer
		// material and texture
		var meshCollider = hit.collider as MeshCollider;
		if (meshCollider == null || meshCollider.sharedMesh == null)
			return;
		var mesh : Mesh = meshCollider.sharedMesh;
		var vertices = mesh.vertices;
		var triangles = mesh.triangles;
		// Extract local space vertices that were hit
		var p0 = vertices[triangles[hit.triangleIndex * 3 + 0]];
		var p1 = vertices[triangles[hit.triangleIndex * 3 + 1]];    
		var p2 = vertices[triangles[hit.triangleIndex * 3 + 2]];   
		// Transform local space vertices to world space
		var hitTransform : Transform = hit.collider.transform;
		p0 = hitTransform.TransformPoint(p0);
		p1 = hitTransform.TransformPoint(p1);
		p2 = hitTransform.TransformPoint(p2);
		// Display with Debug.DrawLine
		Debug.DrawLine(p0, p1);
		Debug.DrawLine(p1, p2);
		Debug.DrawLine(p2, p0);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        RaycastHit hit;
        if (!Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), out hit))
            return;
        
        MeshCollider meshCollider = hit.collider as MeshCollider;
        if (meshCollider == null || meshCollider.sharedMesh == null)
            return;
        
        Mesh mesh = meshCollider.sharedMesh;
        Vector3[] vertices = mesh.vertices;
        int[] triangles = mesh.triangles;
        Vector3 p0 = vertices[triangles[hit.triangleIndex * 3 + 0]];
        Vector3 p1 = vertices[triangles[hit.triangleIndex * 3 + 1]];
        Vector3 p2 = vertices[triangles[hit.triangleIndex * 3 + 2]];
        Transform hitTransform = hit.collider.transform;
        p0 = hitTransform.TransformPoint(p0);
        p1 = hitTransform.TransformPoint(p1);
        p2 = hitTransform.TransformPoint(p2);
        Debug.DrawLine(p0, p1);
        Debug.DrawLine(p1, p2);
        Debug.DrawLine(p2, p0);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		hit as RaycastHit
		if not Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), ):
			return
		meshCollider as MeshCollider = (hit.collider as MeshCollider)
		if (meshCollider == null) or (meshCollider.sharedMesh == null):
			return
		mesh as Mesh = meshCollider.sharedMesh
		vertices as (Vector3) = mesh.vertices
		triangles as (int) = mesh.triangles
		p0 as Vector3 = vertices[triangles[((hit.triangleIndex * 3) + 0)]]
		p1 as Vector3 = vertices[triangles[((hit.triangleIndex * 3) + 1)]]
		p2 as Vector3 = vertices[triangles[((hit.triangleIndex * 3) + 2)]]
		hitTransform as Transform = hit.collider.transform
		p0 = hitTransform.TransformPoint(p0)
		p1 = hitTransform.TransformPoint(p1)
		p2 = hitTransform.TransformPoint(p2)
		Debug.DrawLine(p0, p1)
		Debug.DrawLine(p1, p2)
		Debug.DrawLine(p2, p0)