I hate the term "throuple." And I have not seen "throuplet" used, but that sounds even worse.
Throuple seems to imply that poly is "couple+1" with all 3 being involved. But a triad is 3 Vs stacked up. Say it's 2 women and one man, both women are bi. You have 3 dyads:
M+F1
M+F2
F1+F2
Then you have M+F1+F2.
All 4 relationships need to be nurtured. One on one dates and sex should happen. If it's always seen as "original couple +1" it's doomed to failure. That is couple-centric thinking, and makes F2 seem like a mere addition to the original couple, secondary, and doomed to only have 3way sex and dates, whereas the original couple can have all the one on one sex and dates with each other that they want. They can get together and talk about the unicorn behind her back, and make plans for her without her consent. She might feel lesser and think she has to agree to whatever the couple says. She may not be allowed to have one on one time with either of her partners, not even be able to privately text one of them, without the other member literally reading the texts over his or her shoulder
Unicorns are often abused and seen as a mere sex toy for the original couple to use in many ways. For sex, for housework, as a babysitter. She is left out of the larger family circle, unrecognized as a full partner, uninvited to work parties, with no legal protections, basically no rights whatsoever, and often seen as an interloper by the other woman, with much jockeying for position and status. If she moves in, she may not have a room of her own, or the right to decorate the house, bring her own things there and put them around, take charge in the kitchen, etc.
There's a millennia long history of polygyny (harems) in human culture. It almost always leads to pain for the women. Men are often terrible at managing negotiations between 2 women.