A difficult thing to measure; depends on how you value winning all 4 slams, slamcount, WTFs, YE nr1s, other titles value.
There are plenty of arguments that can be made either way around. I say it is not entirely decided as of yet how to rank these 3, but Djokovic is 2 slams away from definitely passing Sampras and Nadal is probably 1 slam away from doing the same.
Rafa's weak record indoors is definitely a big black mark against him, but his astonishing number of Masters 1000s more than compensates, imo. Plus he does have one more Slam. The imbalance in Rafa's Slam record (10 out of 15 at Roland Garros) is more than evened out by his achieving the career Grand Slam, something Pete never came close to doing. I'd say that Rafa surprasses Pete by a smidge.
Novak is still an eensy bit back because of the 14 to 12 Slams. But if Novak ever gets to 14, then he is a clear step ahead of Pete because Novak achieved the career Grand Slam plus four in a row, not to mention his much higher total of Masters 1000s.