Tell him how you feel. Be upfront about it. You'll only be left wondering what might have been if you don't tell him. You've been friends with this guy for 3 years and are pretty close but don't really hang out as just the two of you but more so as part of a group is that right? You say as well that you liked him before, but now you're falling in love with him. I mean, it might be a lot for him to digest all in one go especially if he's not expecting it. You could be taking a risk, but it's better to be direct than not to be. How do you know for a fact that he's not interested in you? He might feel exactly the same way as you do. You won't know unless you ask him about it so do that.