Transition current camera rotation to 0,0,0

Try this:

 var targetRotation : Vector3;
 var progress : float = 0;
 var rotateSpeed : float = 1.0;

 function Update() {
        transform.rotation.eulerAngles = Vector3.Lerp(transform.rotation.eulerAngles, targetRotation, progress);
        progress += Time.deltaTime * rotateSpeed;
     }

You would have to reset the progress value when the rotation is finished and so on, but this is a base to work from.

Hope it helps :)