Its not easy managing a triad no matter what the dynamic. I am in a similar situation as you and still finding my way through it. My wife and I are deeply commited to each other. Our GF has a loving romance and NRE with my wife. Our GF and I are in love also, but have just started exploring our own dyad intimacy. And we three have an intimate relationship, though it seems that lately its has been far and few times. Some of that is situational and some of that is difficulty with feelings. I usually experience compersion, but sometimes I don't and that is hard for everyone. I think it's not always easy to see the two you love being so into each other, and feeling like you are on the outside. My best advice....communicate communicate communicate. We use verbal communication and an online journal to share our feelings and thoughts. Just because you have the thoughts of being flexible doesn't mean the actual action is easy to do or maintain. It's easy to get hurt or be misunderstood. Keep those lines of communication open and don't be afraid to ask for what you need...not just what you want.
Best of Luck to you all.