Short answer: I have to echo the advice of many others: "Find another time/location when there's no third wheel and no time constraints, be confident in yourself but not smug (unless you're smug naturally, then be yourself), and Good luck!"
HOWEVER, I've done this sort of thing once before, and it's why I'm married today. Not everything went to plan, but the general idea got across. I can't resist a short re-cap...

I'd planned it to be all sorts of dramatic... It was winter break in the dorms. She was leaving for France the next morning. I had come back to school solely to talk with her. I brought a car, something which neither of us usually had access to during school time, and that was my hook. I called her up, saying, "Hey, I got a car, let's go
drive!" Unfortunately, she was with a friend instead of alone, so we all went out together, a friend of mine included too (so I wasn't outnumbered). We drove out and looked at Christmas lights for an hour or so, then came back.
We said bye at that point, but I hadn't gotten to talk to her about what I REALLY wanted to talk to her about... I had mentioned that I wanted to give her a mixed CD for making the flight more entertaining, and so asked if she wanted to come over and pick it up later. When she came by later that night, we talked a bit, then I went and said everything I wanted to say. We ended up talking through the night into morning, and she only left after her friend called to say she was there to pick her up to take to the airport.
Needless to say, the night was a success: we had agreed to start dating when she got back for the next semester. While she was away for that month, we chatted online and sent e-mails back and forth, and then I arranged to pick her up (flowers in hand) at the airport when she came back into town.
Now, my more lengthy advice to you... the things that made this work for me were:
1. We were friends, comfortable together and accustomed to hanging out.
2. Our attraction was mutual, even though we'd both been hiding it (or attempting to), and we were both single.
3. I didn't come on too strong (which could've scared her off), nor was I wishy-washy about it (which could've likewise driven her away). I was assertive and direct about my feelings. I don't remember everything I said, but it doesn't hurt to have a plan in advance, which is what I did. NO NOTECARDS THOUGH.
4. I avoided stupid clichés when talking about my feelings. (No "I want to carry you away on a white stallion!1one" or "I could gaze into your eyes forever <3" lines.)
So, In your case, it's more complicated because she's dating someone, but at least he's a jerk. Just sincerely and confidently be yourself, have a semblance of a plan, and you should do fine. Whatever happens is what's supposed to happen. If you give it your best, at least you'll know and can get on with life whatever the result.
I hope this helps. Good luck to you, Jon. You'll have to let us know how it all goes!