Most Helpful Girl
If you really want his name tattooed on you, wait until you're married (but even that doesn't guarantee that you'll be together forever).
Even if you've been together for almost 3 years, it's incredibly naive to think that you're going to be together forever. Most teen relationships don't last, and 3 years isn't very long in the whole scheme of things. I know you probably think that your relationship is special or different, but that's what most teens in relationships think. The truth is that the two of you are going to change a lot. You're going to grow and mature, your goals in life are going to change---and often those changes mean that you and your partner will grow apart. You aren't going to be the same people that you are now forever.
I had a friend who got his girlfriend's name tattooed on his chest. Now he's married and has children with another girl---and has an ugly tattoo on his chest that he got to cover up his ex-girlfriend's name. I told him it was a stupid idea, but he was head over heels for the girl at the time and didn't listen. Of course, he regretted it later.
Anyway, bad idea. Don't do it.