Welcome to our forum.
From your description of the situation, it doesn't sound like there's enough communication (and not good enough quality of communication) going on. You should be able to express your needs, and feeling of being left out, and you need to know that they are hearing you and listening to you. I think you will probably have to out yourself to the kids soon. Even pre-teens are sharper than we adults usually realize, and your teens are likely aware that *something* is going on.
You can't force your companions to do these things for you, you can only ask. At some point you may have to ask the really tough question: Are you being treated fairly? and if not, what will you do? You can't choose other people's actions, you can only choose your own actions. But make sure you do communicate a lot. If they won't communicate, at least set the example for them.
I suspect they are a little caught up in their own enjoyment of the arrangement, and aren't aware of (or thinking about) your struggles with it. The types of struggles you're describing usually worsen over time, unless all three of you get on the same page and work as a team to help all three persons get what they want.
Are you allowed to date, in addition to being with this couple? It's one potential way of taking care of yourself, if you're allowed. Even if you're a secondary, you have rights. Don't let yourself be put in a position where your needs are always "second place." You deserve better.
Can you tell me more about your situation? Any little detail may help.
I hope some things get better for you soon.
Love means never having to say, "Put down that meat cleaver!"