Off the top of my head:
- It's always OK to express your discomfort. Hopefully a constructive, non-accusatory way can be found BEFORE you reach a breaking point and snap in a completely inappropriate time or damaging way. Express yourself even if you're not sure that what you're feeling IS discomfort, maybe not sure what it is at all, just make sure to explain that confusion/uncertainty is part of what you're feeling.
- It's always acceptable to ask for something you want (though timing can be important!), as long as you're prepared for compromise and possibly even to be told, "No." Communicating about your needs is different. While compromise may be necessary and even valuable in the short-term, in the long-term you can't compromise on getting your needs met.
- As far as the level of transparency goes, only you and the others involved can decide this. There's nothing wrong with trial periods: "Let's try it *this way* for 2 weeks (or 2 months or whatever, depending on the question) and then revisit the question, see how it went and how we all feel about moving forward." Especially in a first poly relationship, you may not know what you are and are not comfortable with right off the bat, and that's okay. You may not discover a problem until after it happens, but hopefully you can keep in mind that no one is purposefully trying to hurt you. Time is your friend. Going slowly is your friend. A little patience, a little benefit of the doubt, and a LOT of communication are your friends.
From what you've written it sounds like all of you are dealing with a very complex situation amazingly well. I wish you all the best of luck!
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack