I am not poly nor have I ever been in a poly relationship so take this for what it's worth, for some reason your post just spoke to me--
I think you need to figure out if you are jealous because you are comparing your relationship or self to his relationship with her and herself
, or if you are jealous because you don't really deep down want to share.
In the first scenario, you just need to flip your mind and convince yourself that relationships are incomparable and this is why you are able to have two going on that are parallel and equal, so to speak. You get different things out of them and they fulfill different things.
In the second scenario, you will need to determine if this is a hypocritical feeling, in that you are able to be with two but he shouldn't be, or if this relationship no longer feels right. Do you want to practice what you preach? I say that in a totally non-snarky way, just that to be able to receive that ability on your end, you gotta allow for it on his, and I don't mean in a permissive sense, I mean wholeheartedly.
I suspect that some of the roughness comes with witnessing newness in their relationship and you are remembering that newness with your own relationship with him.
This is actually why I could never be in a poly relationship with an already established mate - for me personally, the growing pains of one relationship would always be compared to the exotic smoothness of the other relationship and vice versa with the comfort level. And like I said, I have no idea what's going on, just trying to give you some things to jog your mind with and perhaps figure out the root of the jealousy, because that will help fix it.
And FWIW, I believe jealousy is a real emotion and needs to be dealt with. I also think when you move past it your relationship is usually stronger so I very much hope that you get to the bottom of it and the quad is exactly what and how you want it to be.
Also, things always look way better in the morning, so you may wake up and feel like it's not jealousy as much as being weary about the day/week/situation. Good luck!