Texture Switch after getting shot

Im currently using Javascript to switch between textures on an object getting shot at. The idea is that the first texture will be on the object at the start of the game, then once the player shoots a prefab bullet at the object it changes texture, after that the player fires a second time at the object and it changes to the third texture and then the player fires a final time at the object and it finishes off with a destroy command.

So far I have got the first texture switch to work however after that it goes to the destroy command. Any idea how I can get it to recognise the second texture switch? My script is down below any help would be greatly appreciated.

#pragma strict
var Target1Texture : Texture2D;
var Target2Texture : Texture2D;

function Start () 
{

}

function OnCollisionEnter(collision : Collision)
{
	if (collision.collider.tag == "ammo")
	
	{
	
	renderer.material.mainTexture = Target1Texture;
		
	renderer.material.mainTexture = Target2Texture;
     
    Destroy (gameObject);
	
	}

}

Do this then :

#pragma strict
 
var Starting_Texture : Texture2D;
var Target1Texture : Texture2D;
var Target2Texture : Texture2D;
var Target3Texture : Texture2D;

private var iTexture_Set : int = 1;
 
function OnCollisionEnter(collision : Collision) {

    if (collision.collider.tag == "ammo") {
    
        if (iTexture_Set == 1) {
            renderer.material.mainTexture = Target1Texture;
        
        } else if (iTexture_Set == 2) {
            renderer.material.mainTexture = Target2Texture;
            
        } else if (iTexture_Set == 3) {
            renderer.material.mainTexture = Target3Texture;
            
        } else {
            Destroy (gameObject);
        }
        
        iTexture_Set = iTexture_Set + 1;
    }
}