I don't know how well you actually know your crush, but because he's in a relationship, in love or not, to keep from hurting your best girl friend, or your crush, you shouldn't try to interfere. I know what it feels like to think you're in love, but until you've been in a long, serious relationship for several years, it's hard to know what love really is. (you may have, I don't know your dating history, I apologize if I make a mistake) When you think you love someone, it's usually more of a lust. Try not to interfere with their relationship. If they break up in the future and you still feel that way about him, go for it! But until then, don't risk losing your best friends over feelings that may not be there. (again, I apologize if I'm wrong.) Good luck!
If you were really their friend you'd understand they're happy together and don't try to ruin anything! Let your crush know about your feelings though, letting that person know that you know that even though they are in a happy relationship you still feel the way you do. Just come out completely and don't forget a detail. Keeping it from them can really hurt you more than you think.
I was in the same position a few months ago. My two bestfriends had dated and at the time I was happy for them, but I couldn't help thinking that I would be a better girlfriend. After only 1 month they broke up, but we all were best friends (3 girls and him) then I realized he had feelings for me and vice versa, but we didn't date for a while because my other friend was a little jealous.. But eventually we got closer and went on a couple dates and we've been together for a while now! I love him with all my heart and I feel like I can tell him anything, and trust him because he was my very best friend for 2 years! He knows everything about me and I him. Dating your best friend is a great thing :) if your friends eventually break up, maybe you should be a "shoulder to cry on" ;) if you have the same mindset as I did, then their relationship won't last long, I promise. Best of luck! Xoxo