I too still searching for it. There are Femdom plots but that does not fulfil the MMF bisexual thing.
There isnt so far that does, I did find one that does involve two guys and a girl but not really MMF and the female pegs both males in that one. There is a one shot manga called the threeway but the plot in that was more Yaoi and went away from bisexual as possible
The closest one I’ve found was “we started a threesome” where two guys are in a polycule with a girl and one of the guy admits to liking the other guy and it seems the feeling sight be mutual but it hasn’t been updated since . There’s also “love me to death” that is pretty good and ongoing.
I have yet to read a WEBTOON that has bisexual MMF stuff and polyamory and open relationships that actually work are yet to be seen in WEBTOON at least what I have read