I personally doubt the existence of Socrates very strongly and am somewhat mystified why it's accepted that he existed. I actually think he was either a character invented by Plato or based on someone but didn't really do all that stuff.
However, i think you're missing the point. It isn't important whether Plato and Aristotle existed, although i have little doubt that they did. There is a consistent body of ideas and texts written by someone or several people in each case, which can be studied, analysed and debated. Concerning Jesus, if you believe in the atonement, the Incarnation and so forth, he has to have existed for any of that to happen. If you simply accept his words, according to mainstream Christians, you've missed the point. For Christianity to be valid, Jesus must have existed or there is no salvation. If Plato and Aristotle didn't exist, the work attributed to them stands on its own merits. As it happens, Christ's words do too, but Christians don't interpret his life in that way.