I've recently started seeing an already established couple, my first time doing any sort of poly stuff. While I like both of them and don't mind being sexual/affectionate towards Person A, I really only want to be in an actual relationship with Person B. Idk how to bring this up without it seeming like I dislike Person A. I just feel like Person A and I don't mesh super well romantically and if we tried to actually date each other then I'm worried it would end badly, and I don't want that to happen because I like Person A as a friend and don't want to lose them or Person B.
This might turn out to be a non-issue. If you’re okay being sexual and affectionate with Person A, and that’s the kind of relationship Person A wants with you, then you’re all on the same page. Whether you feel like internally labeling it “an actual relationship” is less important than whether what you’re doing is working for everyone.
You say that “if you tried to actually date each other then…it would end badly,” but also that you have “recently started seeing” them - so maybe you’re overthinking this, and the way you’re currently relating to each other is working fine. You’re anticipating a change (from whatever you’re doing now to “an actual relationship”), but why do you think that change is necessarily going to happen?
The best way to figure this out is to ask both people, probably during a handful of low key conversations, both one-on-one and together, what they want and expect from you. Are they envisioning a “balanced” situation where you’re dating both of them as a unit, or are they okay letting your relationship with each individual develop in its own way? What does Person A expect from you?
Then, once they’ve explained to you what they want, you’ll have a better foundation for your response. If they’ve brought up anything that isn’t what you want to do, then let them know that the expectations are mismatched. Instead of saying “I don’t want to actually date you,” try defining specifically what types of things you do and don’t want to do with this person. Then figure out if those are things they want from you. If they are, you’ll need to figure out how to re-set expectations and either find a compromise or conclude that this won’t work for you. If they aren’t, then congratulations - you don’t have a problem!