I am in a somewhat similar situation to the one you are in my friend, but while I am no dating expert, I do know enough to know that pouring your heart out to this chick and letting her know you have been pining for her for three years would be deadly. All you'll do is drive her off. Odds are she sees you only as a friend. Brace yourself for the possibility that she will never see you as anything else. That, sad to say, is most likely to be the way of it.
If there is any possibility of changing this perception of hers, it will take patience and time (and probably still won't work, but since you'll probably always regret it if you do nothing, you may as well go ahead and make the effort, just be realistic, and be ready to walk away from the whole thing if it becomes clear that it's a lost cause). After all, who knows, even though the odds are definitely against you, you just may prove to be the exception to the rule.
As far as a plan of action goes... Well, you need to disappear completely for a while. You didn't say how often you see this chick, or what situations you see her in, whether she's in your class, lives in your dorms, or what, but whatever the case is, be unavailable to her for a while. Stay away. She needs some time to miss you (assuming she will, in fact, miss you, rather than just not even knowing you exist). For Valentine's Day, well... that may be too soon to make any move, again, I don't know your situation, but if you are going to do anything that day it must not be too big a gesture. Depending on your situation, it might be possible to give her something small, such as a card, a few roses (not a dozen), whatever, but which she will come to you to thank you for, and when she does don't say anything about your feelings. Just start acting like a potential boyfriend would act. Flirt. Bust on her with ****y and funny humor. Look her in the eye and also check her body out (respectfully, one time -- don't ogle). Tell her if she wants to thank you properly she should have a cup of coffee or something with you -- no big elaborate date yet. This is why you need to disappear for a little. If you are hanging out with her all the time and suddenly you start acting like a suitor instead of a friend, it'll probably freak her out. A little time apart from her may enable you to make this switch more successfully -- if you have any chance of success at all with this girl, that is.
If she responds well to any of this, you may indeed have a chance with her. If she doesn't, well... your best bet is to move on and work on other women. Odds are you're done at that point. But, if she sees you having fun with other girls, and they're obviously into you, maybe (and it's a big maybe) she'll think twice and start to find you more attractive, but odds still are that you're simply out as far as this girl is concerned.
But if she does respond well, and you get that coffee date, keep up ****y and funny humor, keep acting like a boyfriend instead of a friend, resist the temptation to call her or hang out with her all the time, or I guarantee you she'll get bored and move on. You must remain a challenge and she must never, never suspect that you would crawl five miles over broken glass just to clean the her bathtub with your tongue.
Oh, and read the DJ Bible on this site. I'm still going through it, and not only is it full of extremely valuable information that will help you (and without which you are almost certain to fail), it's entertaining to read as well.
