You should probably verify this, but I don’t think Ghost 3.0 supports built-in comments.
The original idea thread about Membership & Subscriptions did mention adding a built-in commenting system. Not sure what the status of that is, but if “no built-in commenting” is still a deal-breaker, you might still get it in the next release or two 
Ghost does support adding in other comment platforms like Discourse or Commento—but I’m guessing from your earlier post that you know the options and found them unsatisfactory? Is there any specific reason you want built-in comments?