1) This is not polyamory. This is cheating. For polyamory to work, it has to be about openness and honesty. Because he cheated, your husband has proved himself to be lacking both of these qualities, which leaves him massively underqualified to be in a polyamorous relationship.
2) Your "friend" obviously isn't. Anyone who is part of an open or polyamorous marriage should have the good sense not to go behind the back of a married woman. This is very amateurish, and I think belies certain weaknesses in HER marriage. There's no way that their open marriage is structured on a basis of love, respect, and openness that is the core of successful polyamorous relationships.
3) You need to take some time for you, so you can develop your self-esteem. I see in you a woman who is wonderful and giving--sacrificing everything, including your own happiness, for your husband's. The fact of the matter is, he's taking you for a ride. You don't deserve to be treated this way, and you need to stand up for yourself and demand what you want and need from your marriage.
4) You and your husband need to take the time to figure out something that will be mutually beneficial, something that will make both of you happy. Polyamory does not seem to be that thing, at least not for you, at least not right now. You agreed to a threesome--which is NOT the same thing as agreeing to an open marriage. Communication is key here--and if you can't figure out a way to make your husband happy short of letting him go around with a mistress (which is what it is--I refuse to accept this arrangement as a true polyamorous relationship at this point) maybe you need to go your separate ways.
My heart goes out to you because I can sense the depth of your pain. But if you're being forced to give up a monogamy that makes you happy and safe and comfortable in your relationship, maybe the relationship isn't everything you need.
Good luck, and be well. <3