The trio all have one grand slam title to their name this year and it would be a surprise if the winner in New York was anybody else.
Murray will defend a slam trophy for the first time after his breakthrough last year but is likely to have to beat both of his main rivals after dropping to third in the rankings today.
Nadal's stunning form on North American hard courts, with Masters titles in Montreal and Cincinnati, has taken him above Murray and to within touching distance of current number one Djokovic.
Given he has no points to defend for the rest of the year, it appears a matter of when rather than if Nadal will regain the number one spot, but McEnroe thinks whoever has two slam titles can consider himself in the ascendancy.
The four-time US Open champion said: "There's three different guys, obviously Rafa, Novak and Andy, that have won the three majors, and all three of them feel that if they win the US Open - I think - that they're going to deserve to be called number one for the year."
Murray took a break after his brilliant Wimbledon win and returned to action two weeks ago but failed to hit the same heights in Montreal and Cincinnati, winning only three matches in total.
The year's final grand slam begins next Monday, and McEnroe, who will be in London for The Statoil Masters Tennis at the Royal Albert Hall in December, said of Murray: "It's a different type of pressure.
"On the one hand he's done it so that's amazing, on the other hand then you're the hunted more than you were in the past.
"And if you combine that with Wimbledon, this is the first shot where he's got a legitimate chance to finish number one and be number one for the year if he can get two majors.
"So that's a different type of approach and people are going to be gunning for him even more. I think he's found that a little bit since playing Wimbledon.
"But he's much tougher to beat in a best of five than he is best of three, so he'll be ready and I know that after winning Wimbledon and getting ready to defend the US Open, that he's trying to peak at that moment, so it's going to be interesting."