Aack! Just typed a huge response, and my computer ate it.
What Loving Radiance said. Ditto. Your feelings are valid and okay, and it's what you do next that matters.
You really need to tell your partner. He deserves to know who he's marrying, even if you decide you want to stay monogamous. Imagine if he found out twenty years from now that his wife was once in love with two other people and feeling tortured about it, and she never told him? Yikes. Hurtful.
Even if you're scared, you have to tell him. If you can't or won't, then what does that say about the trust you have for each other? If you can't weather this together, you're going to have a rough marriage. But I sense that you're strong, and you can do it.
Now, your friends..eeek! I do not envy that situation. Very complicated. You'll have to decide if it's worth risking the friendship to tell them. If your friend is straight, it is highly unlikely that she'll return your feelings. You risk losing her completely. However, if your feelings are so strong that you might one day have a couple of glasses of wine and end up blurting out your feelings for her, or her boyfriend, then you might want to just bite the bullet and do it. But it's up to you.
You just need to be ready for the possibility that even if by chance she IS attracted to you, she might not be ready/willing to admit that to herself or anyone else. And if she does, she still might not want to share her boyfriend. So yeah, that situation will be even trickier than the situation with your boyfriend.
Just try to be as fair as you can to everyone involved, including yourself. You have to decide if you can live your life monogamously, if your fiance doesn't want to go poly. Don't force yourself into a lifestyle that doesn't feel natural for yourself, or someday you might look at your fiance (husband) and resent him, even though you're the one who made the choice. You also want to consider, if he DOES want to try poly...are you willing to share him?
I, personally, cannot share my husband. I know I'm just not made that way. I would never, ever be doing poly right now if I thought he wanted to be open, too, because I know I can't handle that, and it wouldn't be fair for me to say "hey, I can have a boyfriend, but NO WAY can you have a girlfriend." I'd just stay monogamous instead. But then again...I feel at this point that I'm poly by choice, not by nature or need (I'm still discovering myself). It sounds like you might be naturally poly, to have fallen in love with two people, while loving your fiance. So you have a lot to think about.
Whatever you decide, make sure it feels right for YOU, and be as fair and open and honest as you can be. You might go through some serious "growing pains", but if you are open and honest, I think in the end you'll be living the life you were meant to be living--whatever that is--and be much, much happier in the long run.