Tell her this is a big opportunity for you, which will change the path of your life (for the better) and you cannot pass up on it.
Also say, if you're strong enough (or "meant to be", if she's into the whole fate thing) three years at Uni will not be a problem, and on the other side you'll be stronger.
Me and my girlfriend (will be 18 months into our relationship when I leave so not as long as some in here) have discussed it, and she knows how much it means to me, and that it's more important than us two being able to cuddle every day. If she doesn't understand, perhaps you need to question if your relationship is the correct one? I sure wouldn't let anyone stop me going to uni, sex or not!