They all belong in the same tier great. While Nadal is the only with the career slam, but he dominated only on one surface, but Borg and Sampras dominated on 2 surfaces. Sampras never won the FO, but at least he won 5 WTF while Nadal never won one, and Borg won 2. Under the ATP, all three players have won 64 single titles(Borg 77 if include non-ATP events).