Ask the man on the left "Are you Don?" then "Are you Ron?"
If he answers "yes" to both questions, he's John and the liar.
If he answers "no" to both question, he's John and tells the truth.
If he answers "yes" to one and "no" to the other, ask him "Are you John?".
If he says "yes," he's the liar and his name is whichever one he said "no" to.
If he says "no," he tells the truth and he is who he says he is.
So at this point, you've used either two or three questions to find out the name of the man on the left and whether or not he tells the truth. If he tells the truth, ask him about the man in the middle; if he's the liar, ask the man in the middle yourself. Either way, pick one of the two names that doesn't belong to the man on the left and ask if it belongs to the man in the middle. Since you can trust the answer, that'll tell you the names of both remaining men. So, a max of four questions.