class XXX : MonoBehaviour
{
    Plane plane;
    Transform target;

    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            var ray = Camera.main.ScreenPointToRay(Input.mousePosition);

            if (Physics.Raycast(ray, out RaycastHit hit))
            {
                target = hit.transform;

                plane = new Plane(-ray.direction, target.position);
            }
        }
        else if (target != null && Input.GetMouseButton(0))
        {
            var ray = Camera.main.ScreenPointToRay(Input.mousePosition);

            if (plane.Raycast(ray, out float enter))
            {
                Vector3 hitPoint = ray.GetPoint(enter);

                target.position = hitPoint;
            }
        }
        else if (Input.GetMouseButtonUp(0))
        {
            target = null;
        }
    }
}