I can share a relationship configuration I am a part of. Me and my SO met a couple 3 years ago. She is bi and was interested in having a woman in her life. I was looking for a girlfriend too at the time but my SO and I were meeting a lot of people at the time, and to be honest we had certain things we were looking for, so we didn't follow up with them. I saw the couple at a club 2 years later and found that I was attracted to both of them. I still didn't have a girlfriend so I asked if she would be interested in pursuing that. I consider myself to be in a "dating" triad that is still forming with this couple AND she is attracted to my SO also, so the four of us have done things together too! There is an understood hierarchy of the different relationship configurations. I would put them in this order of importance and this also represents the amount of time we devote to the relationship configurations.
1-She and her husband
2-Me and my SO
3-She and I
4-She, I and her husband
5-She, I, her husband, my SO
6- me and her husband
These configurations will probably never happen
She, her husband and my SO
She and my SO