I feel for you, this is awful. Polyamory is not supposed to be like this. It is supposed to be about honesty and respect. They have cheated and lied to you and you have every right to feel betrayed, angry and six kinds of terrible. The fact that you are finding some hope and good feelings about this situation says that you have a bigger heart than I could likely muster in that situation.
I'm sure that a number of more experienced people will weigh in, but here are a few things that strike me about the situation:
-- As the harmed party, you have every right to ask them to put their screwing on hold for a few weeks while you get your emotions under control. If they are unwilling to do this, I would seriously be wondering if you want to spend much of your life with these people.
-- You seem to feel that you have to do this. As RedSirenn, points out, you do have choices. If you habitually allow yourself to be disrespected, people will take advantage. A very important longer term goal would be to do things to gain a more equal stance with your husband. Hard to say anything more specific since I don't know your situation (economic, etc.) in any way. If there is a massive power imbalance in your relationship with your husband, adding more people to the mix will just make things worse.
-- It really burns me that they did this when you were trying so hard to get into this polyamory thing. You had already moved far from your comfort zone in allowing 3-somes, etc. Their behavior was a massive disrespect to the compromises for the relationship and courage you had already shown. An old girl friend did something like this to me years ago. It wasn't just that she had screwed another guy, but the cruel way she treated my feelings. I dumped her.
-- Ideally, a polyamorus marriage starts with a very strong relationship to begin with. Opening your marriage will put huge strains on your existing relationship with your husband. I am very uneasy about the health of your marriage now. Can it take the strain of adding more people to the mix, particularly ones who are amoral?
I'm afraid I've mostly looked at the bad news. However, a few successful poly groups have formed from starting points as bad or worse as yours. (Some of them are posters on this forum.) So if you are determined to make it work it can happen. But the others have to start making an honest effort if you are to have any hope at all.
Please take care of yourself.
Warm regards, Rick.