What if I like one partner more than the other?
It's common, and it isn't a failure on anyone's part — connection is rarely perfectly even across three people. Say so honestly rather than performing equal interest you don't feel. You are never obligated to force equal feelings to keep an arrangement comfortable for the couple.
Attraction doesn't distribute itself evenly just because a couple hoped it would. Expecting a single woman to feel identically about two different people is a fantasy convention, not how connection works — and most couples who have genuinely thought through a third joining them know this.
The honest move is saying so, to both of them, rather than performing equal enthusiasm you don't feel. Faked symmetry tends to surface later in ways that hurt more. A couple who talked through this possibility beforehand will hear it without treating it as a crisis; a couple who reacts with anger or blame is telling you something about their preparation.
What you don't owe is the emotional labor of fixing the imbalance. You are not responsible for reassuring the less-favored partner, mediating between them, or shrinking your interest to keep the scales even. When an existing couple's comfort is consistently placed above your honest experience, that's couples privilege — the red-flag patterns are laid out in the unicorn vs throuple guide.
From there, several outcomes are all valid: the dynamic continues with uneven attention (some couples are genuinely fine with that), it pauses, or it ends. Ending it needs no more justification than "this isn't the fit I thought it was" — declining or stepping back never requires an explanation that satisfies anyone but you.