My partners let everyone think it's just them in a monogamous couple, and it bothers me

I’m in a serious relationship with two people ( I should say all three of us are dating each other). They were dating for a few years before, and I’ve been dating them for a few years now. Any time they come out as having a partner, they always come out as dating each other and it’s never me. I understand if they don’t want to come out as poly/having two partners, but it sucks that I’m always the one that has to be the secret partner, the one who has to keep to themselves in public. We’ll be out together and they can act like a couple in front of coworkers and family while I’m like a third wheel. It makes me feel so bad about myself, like I’m not equal in the relationship or feeling more like a side piece. I know if I said anything they’d probably just tell me “that’s just how it has to be.” Am I silly for feeling bad about this? Or is this unhealthy? I don’t want to break up because I love them but I’m tired of being the dirty secret. This is my first poly relationship so I’m not sure if this is normal, but I know it doesn’t feel good.

Your critical error here is assuming that they’d say “that’s just how it has to be” before you even try bringing it up with them. You’re not psychic - you can’t read their minds or see the future. Talk about this! Mention that you feel bad having to keep your place in the relationship so secret, and see if they have any thoughts, feelings, or ideas that you didn’t expect. It’s possible that they never really thought about it, or assumed you were okay with it, or even that you prefer things this way.

If they do say that this is how things have to be, and aren’t willing to talk about whether, where, and when it would be safe to come out as polyamorous and ask their friends, family, or coworkers to accept you as their coequal partner, you have to decide whether staying in an arrangement that “doesn’t feel good” and makes you feel like a “dirty secret” is worth it to you. But that is a bridge you definitely don’t need to cross until you know it exists!