How to access gameObject variable script

You can do it two ways:

Either the script does it on it's own on every object using OnMouseEnter, OnMouseExit and OnMouseButtonDown

 bool mouseIsOverObject = false;
 bool thisObjectsCounter = 0;
 
 void OnMouseEnter()
 {
    mouseIsOverObject = true;
 }
 
 void OnMouseExit()
 {
    mouseIsOverObject = false;
 }
 
 void Update()
 {
    if(Input.GetMouseButtonDown(0)&&mouseIsOverObject )
        thisObjectsCounter++;
 }

or

You have a script on your camera that uses raycast, raycasthit to get the hit object; your 3 objects than need colliders though. That way you get the hit object, then you can access it's script with GetComponent and then add one to your particular variable.

 Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
 RaycastHit hit;
 if (Physics.Raycast(ray, out hit, 100))
 {
    if(hit!=null)
       hit.collider.gameObject.GetComponent<MyScriptClass>().thisObjectsCounter++;
 }