That's a hard spot to be in, and there isn't really an end-all solution that'll get everyone what they want. But, based on what you've said, I would recommend not interfering. Honestly, you have no business in their relationship unless one of them is being abused by it and you feel that safety/health is on the line. If they're happy, as you say they are, then you should be happy for them. Be happy that two of your friends are content and that they have someone close to them.
But never forget about yourself and the predicament they're relationship puts you in. I was in your exact same spot a few years ago when my two best friends started dating. I became the third wheel. I felt unwanted. Eventually I told them that while I was happy for them, I would not spend time with both of them at the same time. It was too painful to feel that cold shoulder from both friends (even though it was unintentional).
Of course, I don't know you, and I can only make this statement based off the fact that, through your typing, you appear to be a bright young lady. My advice to you is to keep your heart open; don't get so wrapped up in a what-could-have-been that you miss a what-still-may-be. The old adage "there are plenty of fish in the sea" is pretty true. Keep your wits about you, be a social butterfly, and eventually things will work out.
