A caveat: Rafa has had greater success on hards than Novak and Roger on clay, but that doesn't in itself mean that he's a greater hard court player than they are clay-courters. But not for reasons that Cali is saying. But this is one area where I think it is valid to differentiate between accomplishments and ability.
But it is undeniable: Rafa has had greater success on hard courts than Roger or Novak on clay. But it is not because there isn't a second clay Slam. It is because Rafa is such a great clay player.
The main problem with assessing anyone (other than Rafa) on clay is that Rafa on clay is the clear "surface GOAT." No one else dominates any surface like he does. Playing in the last decade and a half, you could literally be the second greatest clay courter ever and still not have more than an RG title or two.
I'm not saying that Roger or Novak are the second greatest clay courters ever, just that their single RG titles don't accurately represent how good they've been on clay.
How about, instead, RG finals:
12 Rafa
7 Borg
5 Vilas, Lendl, Wilander, Federer
4 Djokovic
3 Connors, Courier, Bruguera, Agassi, Kuerten
Only two players have reached more RG finals than Federer: the two that are generally considered to be the two greatest clay-courters of all time. Lendl, Vilas, and Wilander all have a better clay resume than Roger, but that is skewed by Rafa. So it is hard to say.
Novak will likely reach at least one more RG final, if not more.