Without Nadal and Djokovic, Federer would have made a joke of tennis.Tennis would have been murdered by Federer being too good without adequate challenge.
Djokovic appeared a bit "late" in Federer's career. But if it wasn't for Nadal Federer would have been the most dominant athlete at any sport and the undisputed goat. We would be talking about a guy having 20+ slams, 2 calendar grand slams, like 4 career grand slams, 10 slams in a row and 500 weeks in a row at number 1 maybe.
Good thing that Nadal appeared. As you said, Federer would have kill tennis if not.
Any of those 3 would have dominated easily tennis without the other 2, but because of his consistency and longevity Federer would have it even easier.