It feels like they are close to their end now. Federer all but finished, Nadal regularly injured and can lose to many players outside RG, Djokovic with many close matches this year which he could have easily lost.
But saying that, Federer probably makes a comeback and wins more slams, Nadal will win RG and Djokovic will romp to victory at the AO. The big 3 always seem to defy expectations when they look close to finished.