Ignoring what it says in the holy books themselves, what evidence is there that any of these three gentlemen actually walked the earth?
Or, to put it another way, even if they were real people - what evidence is there that they were 'special'?
There is no evidence any of them were special.
Probably the one who has the strongest evidence of being a historical figure is Mohammed, as the Islamic religion rose very quickly after his life.
Jesus is a bit more questionable, since we don't get accounts of his life until a century later, and even then, they massively contradict each other, even the four "Canon" Gospels the church recognizes, much less the hundreds they rejected.
I don't know enough about Bhuddism to venture an opinion on Bhudda. However, the fact that the first writings about him don't emerge until 400 years after his death, I'm doubtful.