I don't think there is anything wrong with being friends with someone who is attracted to you - as long as he is not ever bringing it up with you after you've already told him that's not what you want.

If he is bringing it up after you've already told him how you feel, that's just setting yourself up to be hurt - because that means he still thinks there is some chance you will change your mind, and hes hoping for that - that's why he would be telling you. "I like him I really do" "its been 3 years" "I've told him many times how I feel" So the dude obviously wants you, he is telling you this because hes hoping you have changed your mind. He needs reality check.

When he realizes that is not going to ever happen, he will either

• A. Continue to be your friend, and get over himself - which is what any person who was actually a friend would do, the best outcome for both of you. Wouldn't you want to be able to tell a friend you're close with that you love, that you love them - without it being misinterpreted?
• B. Stop being your friend, because he was never your friend, and just wanted you romantically - A "Nice guy".

If you don't tell him, and you just keep ignoring it, you're only opening yourself up to be hurt if he chooses option B years down the road. Don't do it for him, do it for yourself.