Assuming your BHs are the same wt as your FTs and your FTs are grouping well. And your FTs are shooting in a vertical line (walk back), then I would have to think that your rest needs a bit of tweeking. But 3" at 20 yds makes me think you didn't do any walkback tuing and your rest is slightly out of adjustment OR you are getting some fletching contact. I would check for fletching contact first befor anything. What kind of rest?