One Approve Button for Multiple Approvers

I would suggest three buttons or three sets of buttons (if you need a button to Approve or Reject).  Then have the buttons calculate to blank where they no longer make sense to offer.  Then the button will disappear.

There approval workflow type apps are actually tricky to get working  and be firing the emails correctly to the correct people and then you typically need safety net Daily Subscription emails ot back up the email Notifications.