To me it seems like what you're really feeling is afraid that you are not as loveable or important as the other folks in your situation rather than jealous per se, because as you say, you like your husband's girlfriend.
First of all, in my opinion, I am sure that there are things your boyfriend and husband see in you, since they are with you, and you seem like a smart kind person, even from this one post, so I'm sure that there are plenty of things about you to love.
My suggestion is to look at your feelings toward yourself irregardless of how the others in your life are treating you or behaving toward you. Do you like yourself? What talents do you have? What have you accomplished? Then look at yourself in regards to relationships; what do you bring to the table? What sorts of things are you good at that your partners appreciate? It seems to me that finding where you fit in with yourself, and then where you fit in with your partners will help you see your role in your group.
I don't know you, but I do know that in most poly relationships that work, everyone has a good role to play, and each person contributes a part that makes the whole. Just by reading your post I pick up that you're thoughtful and caring, and smart! There was nothing hateful or mean in what you said, it just seems like you're being honest about your feelings, and that's a step in the right direction.