I will ask you a couple of questions before I give some advice.
Considering this is an anomymous post, do not be rattled or upset with the questions I will ask. Your privacy is protected which is one of the best thing about this forum.
1.
What is your height and weight?
2.
What is your ethnicity?
3.
Rate yourself on a scale of 1-10.
10 being most attractive
5 being average
1 being very unattractive
In regards to the 3 year crush issue, I have to ask for elaboration on the matter.
Asking out your crush without any previous friendships with your crush at all will 99% of the time fail. Logically, why would any woman accept an advancement from a man she doesn't know. Unless you are a 10/10 man (which even in that case is unlikely to be accepted), your chances of success is very very slim.
Now lets entertain the idea that you were/are friends with a crush so much so that you have a deep understanding with each other, then there are 1/3 common grounds that could be the case.
Now this is the most common one.
I propose that you ask yourself this question. If you were a girl, would you date yourself? Not based by preference or what not, but by purely looks and body. If the answer is yes, and you believe yourself to be of average+ looks and body, then move on the 2nd point.
1.
She doesn't see you romantically.
Providing that you guys are friends, then she probably sees you as just a friend at that moment of time. Views change over time, so this could be different the longer you guys stay friends. But do not take what I am saying as definitive. Move on from the crush.
This one is self-explanatory. There is nothing you can do in this instance. Just move on.
Now notice how my advice is quite abrupt in the sense of moving on. You must be able to face and accept rejection. Empirically, ~80% of advances are rejected by woman, so the fact that your crush rejected you is not uncommon by any metric. They are many women that will find you attractive, providing you look after yourself and make yourself the best possible version of yourself which is very simple to do. Go to gym, have plans and goals to be successful, take some skin care routines, hair cuts etc.
As for your crush finding someone else, you didn't initiate anything. You even stated that you barely talked to her which indicates that your crush probably doesn't know you very well. Confidence is key and thats what you need to exhibit when looking for girls.
Make sure you answer those 3 questions, will be here to help.