I like what Marcus says about triads. Being in the land of one (just had dinner with our kids intermingling, while the adults worked out date nights and reactions to past dates and all), it's just an incredibly rich, complicated realm. I'm not sure I would have picked this, had it not happened. But it's happening, and it's more than three couples, really. There's A-B, B-C, and C-A, plus there's also the actual triad of A-B-C (where we recognize that we're subtly different when we're all three together, in terms of talking styles, sex, and so much else), and then there's also A, B, and C as solo individuals outside of each of the couples, and interacting with and reacting to the couple. That makes for a lot of relationships. In a "typical" couple there's just A and B and their interaction as a couple separate from their interaction as individuals. Triads are complicated.
And they're awesome, when they work! Holy shit, is this fun. BUT, I can't imagine having expected it. What's happening isn't what I expected even two weeks ago. We're on a big web, and when one person moves, the other two move, and all the relationship interactions subtly change. There's no way to predict it or plan for it. The only trick we've got going is plenty of communication and a deep space of trust.
What you've described sounds like affection and intimacy, growing into something more. That sounds wonderful, and can happen. For us, it happens best when the individual (A, B, or C) is spending plenty of time supporting the couple they're not in. If we all trust that we're all doing that, THEN it works out. A complicated bit of game theory, really, and some triad version of the prisoner's dilemma, but for good and not for bad.