I don't know about Muhammed or Buddha, but the historicity of Jesus can certainly be questioned.
The fact that many historians who lived at the same time as Jesus in the same area with the same interests did not mention a man performing incredible miracles claiming to be the messiah did not seem to mention Jesus at all is certainly a dent.
But then we have sources saying they actually was quite a few people claiming to be the messiah at the time of Jesus, and faith healers were actually quite common in those days. So maybe it wasn't that unusual.
The gopsels are unrealiable as hell, because they definitely have political goals, conflicting messages and events - I mean John goes way out and is clearly anti-semitic in tone. The Q document is the best evidence that Jesus was real, this is a number of teachings that is present in Matthew and Luke, but not present in Mark - which means it is seperate from Mark. But the Q document does not include any miracles of any kind, and just have a list of parables. Which means Jesus could have been a preacher, but not a miracle worker. In Mark, Jesus nowhere claims to be the son of God, in fact "son of God" does not actually mean a divine breeding has taken place at all in Judaism. In Judaism, "son of God" simply means close to god - just like King David was called the son of God.