Adam, Noah and Moses were only attested and believed to exist by groups living in the Kingdom of Judah.
For Moses, people have looked a lot for any kind of evidence that there was some significant Jewish presence in Egypt, or Egyptian migration to the area of Israel, and nothing of the kind has been found, either in Egyptian documents or in archaeological evidence - which is actually evidence of absence when expecting a significant population to have migrated that way. Further accounts from the books of Exodus and Deuteronomy, such as the battle of Jericho, have also been somewhat conclusively debunked (the city of Jericho hadn't existed, at least not worth walls, for a few hundred years before the time of the conquest is supposed to have taken place).
Similarly, we have looked long and hard for evidence of a massive flood that could have lent some credence to the story of Noah, and nothing of any significant magnitude was found for that time - and here, we know for sure that a flood would have left significant geological evidence, so we know the flood can't have existed.
Adam has so little information associated with him that it's hard to even define what it would have meant for him to exist. We do know for sure, based on DNA evidence, that there is no single father + mother pair from which all humans living today have sprung, definitely not anyone living anywhere near close to the Jewish account of Adam.
In contrast, the idea of a founder of the Christian sect, one who was killed under Pontius Pilate around the year 33, has no major evidence against it, and is a somewhat plausible account of how the Christian sect could have come to be. There are no sources asserting a different origin, and there are no sources that contradict the possibility that Pontius Pilate and the Jewish authorities would have punished someone behaving like Jesus did. So, the neutral position is to say that he may or may not have existed, we don't know.
If you further believe the biblical or non-biblical sources attesting to his existence, even if you think they are weak, you can even say that it's more likely that he existed than that he didn't.