Not 100% what you're asking, but freshman year of college, mid-1980s, there was a girl-guy-girl 'throuple' from Eastern MA. The two girls' parents are very tight, so the girls keep it civil and casual during high school. In fact, all 4 of said parents don't think the guy is Mr Right--they don't mind the other girl being in the picture--that way, things won't get too serious too fast.
One of the girls (let's call her girl #2, since that's what she had become by that summer) and the guy, stayed in-state for college.
Girl #1 went away to an expensive Vermont private school, 2 miles due north.
Anyways, about 2-3 weeks into Sept., Girl #1 quickly realizes her mistake. The guy is returning her phone calls (this was pre-cell phone) less and less, and even worse, his stories of how he spends his time has inconsistencies--she catches him in multiple lies. She tries to make him jealous by making new guy friends, but the Vermont school is small and its social life is non-existant.
Girl #1 then actually enlists her parents to ask Girl #2's parents to see if Girl #2 is making a play for the guy. Girl #1 gets reprimanded by her Mom that their firendship with Girl #2's parents are important for all 4 of their careers (their company's have a vendor-client relationship, and all 4 earn commissions from that relationship staying active).
So a pair of heart to heart mother-daughter phone calls happen one weekend: Girl #1-her Mom, Girl #2-her Mom, that the 2 girls need to "work out between themselves" their unsustainable throuple situation, and the parents don't want to be involved and don't want to hear anymore about it.
Now, since this was IRL and not fantasy, no, Girl #1 and Girl #2 did not face off in a nude boxing match.
BUT....
Girl #1 DID make 3 weekend road trips down to MA to try to show the guy her prowess in the bedroom.
But his weekday cheating with Girl #2, and his phone call lies, continued. So Girl #1 stepped aside and moved on.