I'm a relative poly-newbie but here's my 2 cents
I don't think there is any rush here even though it sounds like your feelings are intense. It sounds like you're making a lot of assumptions with the couple, maybe with a dash of wishful thinking. I am also concerned b/c, to me, being poly is about being totally honest, honest about who you are, who you want to be with & how you manage your relationships. If you're having problems with your SO, I think you need to start resolving those issues before you look elsewhere for happiness. Again to me, poly is about adding more to your life, not about fixing something that's broken. I'm wondering if maybe you're so starved for some connection b/c of a possibly failing relationship that maybe (and that's just a maybe) you're making assumptions with the couple & jumping headfirst into something that can crash & burn.
With my poly relationships, maintaining my friendship with my lovers is way more important than the romantic/sexual nature of our relationships. I would never want anything to get in the way of the friendship. That's just my situation & my opinion though. Think about the consequences if you've made some incorrect assumptions & the couple doesn't accept your advances? We must always consider the consequences of our actions in all facets of our life. I have 2 little kids & I view this as one of the biggest lessons I can help them learn.
Get honest with yourself & get honest with your SO before you do anything you might later regret. Once you put it out there, you can't take it back.
Me- Bi-female-early 40s- Single Mom to 2 young boys
Exploring & having fun for the time being after a long & unhappy marriage
There is no failure. Only feedback. -Robert Allen