I am one of those fortunate few who has lived poly lifestyle for over 20 years. For me, there have been three triads of fairly long-term duration with bi female partners. Of course there are V type relationships also where one person has primary relationship with each of the other two but those two rarely are sexually intimate with eacdh other.
Please note that polyfidelity is not swinging per se although some swingers are also or become poly. It can be harder to maintain relationship equity but eliminating the desire or need to lie and cheat removes that difficulty as a factor. Jealousy can always be an issue even in poly but it is so much easier to prevent the damaging repercussions when you talk and modify behaviour to address that green monster.
A big plus for this kind of relationship(s) is that it ethically and positively removes some of the communication difficulties. I am currently partnered with only one bi, disabled female so since we both enjoy having another as full part of the family, the search goes on again. Our last LTR ended sexually about a year ago. That woman is still a very close friend.
Poly relationship takes work, time and excellent communication and it is NOT all about sex.