Three years is a LONG time. Would the person even remember the transaction? If you think they would or should remember...then it never hurts to mention the mix up and give them a chance to do something about it if they wish to.
However, you're not really in a position to force them to do anything about it. If they offer some kind of compensation, then you're better off than you were before you said anything. If they don't offer any compensation, then you at least know where you stand with them and are no worse off than if you hadn't said anything at all.
I agree also....take the lesson to heart. Never assume an animal's sex just because an "expert" told you so. Even the best of them can make a mistake sexing a snake. It happens.