how to make 3 instances with interchangeable position

i need to make this for a game. basically i have three objects, and when i press left mouse, the one behind the first pass in front, while if i press right mouse it's the third one that goes in front of the first something like this: http://imgur.com/fMCj38n

here's the code, but that's the big problem:

 #pragma strict
 
 var sphere:int=0;
 var cube:int=0;
 var capsule:int=0;
 var pos1:float=3;
 var pos2:float=0;
 var pos3:float=-3;
 
 private var target:Transform;
 
 var message:String="-ciao-";
 var weight:Rigidbody;
 
 function Start () {
 //    gameObject.name = "Pippo";
     //target = a;
 //    player = gameObject.FindGameObjectWithTag("Player").transform;
 }
 
 function Update () {
     if(Input.GetMouseButtonDown(0)){
     
     transform.position = Vector3(pos3,0,0);
     }
     
     if(Input.GetMouseButtonDown(2)){
     
     transform.position.x = pos1;
     
     
     }
     }

basically, the objects begin revolving on each other, what's the problem?