I would have Fed>Joker>RAFA. But I can see RAFA staying at 20. He's the heavy favorite for RG and if he gets 20 then he will push himself to the brink to get 21+. If that happens I can't see Joker winning 5+ slams to pass him. Fedr needs to win AO 2020 or Wimby 2020 to stay in the lead.
Injuries are the wildcard. Barring injuries it will go Joker>Rafa>Fed. Djoker would have won this healthy, easy. While Rafa lucky to steal one, and Fed, other than at Wimbledon, is on fumes.
Prediction: Rafa gets 2 more FOs. Hits 21. Fed gets no more into the clubhouse at 20. Djoker 2 more wim, 2 more aus, 1 more Fo, 1 uso. Gets to 22 min--if fed and rafa hang it up, could pad that more in a post big 3 world.
You would have to think that Djokovic and Nadal would pick up at least 1 more major between them. Nadal is a shoe in for RG still and Djokovic can still easily win Wimbledon and AO (more so Wimbledon).