Okay, big time red flags there. Understand that while you are frustrated and the two of them aren't behaving ethically or honestly, you trying to destroy the relationship will just make it worse. Simply because then it will be your fault!
My suggestions, such as they are, are to fist sit down with your husband. Explain that with all the research YOU have done, honest communication and trust are important. Ask him to check out some of the research. People here are more than happy to point you to some great stuff! For us, Tristan's book 'Opening Up' was incredibly helpful. We have even relaxed in bed and done some of the little check lists in there. Just to define together what things mean to each of us.
Mention to him that you think it's important not only for the two of you but for all three of you to sit down and figure this out, for everyone. If someone doesn't want to do that, I would be INCREDIBLY blunt and explain that if you aren't ready to sit down and figure things out honestly and openly you sure as hell aren't ready for a poly relationship.
If nothing else, you need to hear, from her, what exactly she thinks this relationship is, what he thinks it is, and all together is best so there's none of this he said she said crap.
Something we have used for our kids was mirroring. Not just repeating what someone else says, but HOW they say it! It really helped one of my children realize that while they thought they were being very mature, they were coming off as rude, eye rolling, and dismissive. I do suggest that is done all three as well just so that hubby can see WHY you are feeling disrespected by her. Actions mean a lot and those little looks and eye rolls might not seem like much to some people, but when they see it mirrored back it's sometimes a shock!
Lastly, hubby needs to understand that yes, it's hard to be a hinge, and not feeling like you have to pick sides in a situation. It's not easy. It's important though to NOT bash one to another. There are things I just keep to myself! It's hard because I like sharing things with both of the men in my life but I remind myself, if it only causes them to feel badly, then why? I'm not talking big things, just little things that might make them feel insecure and question themselves.
(P.S. this did backfire when they met and are not BOTH upping their gym time to get into better shape feeling the other has certain parts that are better. Legs and chest and all that. Not going to mention that *I* am the most out of shape of all three and being told I don't need to work out!)
Me: 40 pansexual poly.
DH: My husband of 21 yrs and father of 3 teen girls.
DC: LDR of +9 years/former