Sorry to hear you are going over some rough ground. Sometimes poly relationships are difficult in the early stages. The one I'm in took a few years to smooth out (and I am partly to blame for that). I can also relate to "feeling second-class." My companions are married to each other, so I felt like the odd man out (or homewrecker) often in the early days. It just gets easier after awhile, is all I can tell you. I don't feel left out today.
I don't recommend bottling things up, and I hope your companions will give you a fair chance to speak your mind. There might be some things you can do too; there might be a more diplomatic way to say things, and a better time and place. It would be a good idea for the three of you to have regular sit-downs, once a month or even once a week, where you can talk about your feelings, needs, and boundaries. Try to steer away from words that assign blame, and instead, talk about your feelings and make requests. Sometimes how you say something is as important as what you say.
Love means never having to say, "Put down that meat cleaver!"