So my partner and I have someone we play with and are very connected to and have been batting the idea around of diving into polyamory with them as our third.
Our third partner has someone they are dating that isn’t involved with us (friends, but nothing other than that). They have an open relationship as well.
So my question is, it is possible to have a 3 way relationship and have that person's [other] partner uninvolved in it? Has that been done?
I have never been romantically involved with my nesting partner's other partners. In fact, by the choice of her men, I've barely been socially involved with them in any way. They have all generally preferred to "host" her, so we rarely interact.
However, I generally host my other partners in our home. So, Pixi has met and hung out with several of my bfs or gfs over the 9 years we've been together. In a couple rare instances, she has shared sex with them a time or three. This generally hasn't worked out, so we've stopped doing it.
Most polyamorists don't even do TRIADS, as you are. If we are coupled, we date independently. We don't expect a "third" to feel fondness/sexual attraction for both of us equally. And attempted triads often do go bust because the shared partner (the unicorn, the "third") either starts to prefer one of the couple over the other, or tires of feeling "lesser" as opposed to the more established couple, or hates causing jealous feelings in one or the other of the couple, or starts to feel as if she is being used, etc., etc.
But anyway. NO, you are definitely not required to take on your shared "third's" other partner as your fourth! God, no. In fact, I'd advise against even trying. Too complicated. Too many moving parts. Triads are hard enough. Quads are worse.