Well, this isn't exactly groundbreaking news, but I will say in favour of Nadal and Djokovic, that they are simply products of the courts today IMO. It seems strange to say, but I think they'd be completely different players had they played in the 80's or 90's. If they knew they needed to succeed on fast courts, they'd play completely differently than what they do now on comparitively slow ones. Their styles succeed in todays game which is all that really matters. Besides that, Nadal would still have 7 RG titles for sure, and he'd probably still have his Aussie title, and Djokovic would probably still have his 3 Aussie titles.
As for Federer, yes there are players who have better serves, and/or vollies, but Federer's played a fair bit of S&V, and he's amazing when it comes to returning big serves especially in his prime, so he'd survive, and he'd probably still lead in GS count IMHO.