I have liked a boy for three years and never said anything, so I have always been friends with him. What are the chances of success if I confess?

You won't know until you confess. Your focus should not be on predicting the probability of success before confession, but on grasping two appropriate words when confession: First, if confession is successful, everyone will be happy. Second, if the confession fails, you won't lose face, but it won't cause harm and embarrassment to both sides, and you can continue to be friends.