Camera.main.ViewportToWorldPoint not working like expected.

Hello everybody,

I am writing a script that Showcases Objects that you click. For example a Sodacan on the floor will be displayed at the screen above a gui that shows some stats and a bit of text + a 3D Model of the clicked Object. Here is a bit of the code i have (without Gui stuff):

 var open:boolean = true;
 var close:boolean = false;
 
 var Inst_Obj:boolean = false;
 
 var ShowcasePos: Vector3;
 var ShowcaseObj: Transform;
 
 
 var GUIStyle1: GUIStyle;
 var GUIStyle2: GUIStyle;
 var GUIStyle3: GUIStyle;
 
 var Player: GameObject;
 
 
 var rotation: float;
 
 
 
 function Update () {
     ShowcasePos = Camera.main.ViewportToWorldPoint (Vector3 (Screen.width/2,Screen.height/2,0));
     Player = gameObject.FindWithTag ("Player");
     
     var distance = Vector3.Distance(transform.position, Player.transform.position);
     
     if (distance < 3)
         close = true;
     else
         close = false;
     
     if (close == false)
         open = false;
 
 
     if (open && Inst_Obj == false){
         
         ShowcaseObj = Instantiate (transform, ShowcasePos, Quaternion.identity);
         ShowcaseObj.rigidbody.useGravity = false;
         ShowcaseObj.gameObject.layer = 8;
         Inst_Obj = true;
     }
     
     ShowcaseObj.position = ShowcasePos;
 }
 

So it works but i have problems with positioning it (the ShowcaseObj) on the screen. So how can i use the ViewportToWorldPoint function to display it in the right Position? When I leave the Y Position on the Vector3 zero it spawns the Object right at the camera. But when I use 1,2,3... it spawns at a completely random position very far away from the camera. What am i doing wrong?

Thanks for the help in advance.

-Alexander