Welcome to the forum. You should find many people here in similar situations. I just wanted to add what Sea posted.
It sounds like you are very aware of the pitfalls, and what might arise, and are willing to try to make it work. Not just for your wife, and her b/f, but yourself as well.
Sea and Tommy, came from the same situation. I have to say honestly, that they thought they would be aware and able to deal with things that arose. They weren't, and neither was I. I thought coming from the swinging lifestyle, that they would be far ahead of my thought process. But swinging is sex. Yes, the ultimate goal for some is to find a couple to share every aspect of your life, but it doesn't seem to get there, unless you are very fortunate. So in walks me. I am single, and entered our relationship believing that what they had learned from their previous lifestyle would give us all direction. I have found that poly isn't like that at all. When you bring emotions, and feelings, and start sharing each others lives, all you believed that you had learned previously, takes on a new learning curve.
Your feelings aren't unfounded. If you don't know. You don't know. It sounds like the three of you are going into your new relationship with open eyes and open hearts. Keep communicating. If you're not sure, say you aren't. If you need a little more time to deal with the current situation, say you need a little more time. If it's real love between your wife and her partner, it won't go away, because you need time to adjust.
Until reading your post, I didn't realize that sometimes one partner or another feels left behind, because we aren't able to share the NRE. We want them to be happy, we also want to share it, and we do. Sometimes only by watching and supporting. That doesn't mean we can't feel a little left out. That NRE is exciting. We remember it, and want it for ourselves as well as our partners. Redpepper said it best. " The more love you give, the more you receive." But that doesn't mean it won't take a little while to get there, and see it and recognize it for what it is.
I hope all goes well in your new found relationship. You realize it won't be easy, and maybe that's all you can do for now. See what could be.