Federer, Djokovic and Nadal are still a class above Murray. Even Federer at almost 34 years of age can comfortably beat Murray on grass. Sure, Murray has beaten them at Grand Slams in the past, but Murray is usually very little threat to them.
When Murray has beaten them in the past, it's not Murray who has played well, it's they who have played below their usual standard. If they are on form, Murray has no chance.