Element 3D musi posiadać na sobie Collider.
Camera.main możemy cachować do zmiennej.

    GameObject activeObject;
    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            RaycastHit hit;
            if (Physics.Raycast(ray, out hit))
            {
                activeObject = hit.collider.gameObject;
            }
        }
    }