I don't think Jamie really meant that there were 3 people that he knew that could've beat him. More like admitting that he was very good and maybe on a few could best him. He does probably think Barristan would be one of them, but I don't think we've actually met any others that would be better (books or show) and still alive when Jamie mentions this. Obviously Dane would have been better than either Jamie or Barristan, but not alive when Jamie said it. So it's more of offering the possibility that others may be better, but very few.