Don't waste your time on this trash. From the novel:
Turns out the ex girlfriend was friends with this "handsome" idol. Handsome idol was childhood friends and liked MC since back then. But when she came back he was already with ex girlfriend.
So brainlet ex girlfriend decided she and handsome girl (which is also her friend from school years prior) should make a bet. Ex gf would pretend she cheated on him and give handsome girl (MC's childhood friend) a month to make him fall for him.
Long story short both girls try to seduce him during that month cause ex gf couldn't keep her end of the deal.
Basically they just used him as a f*cking plaything the whole time while he was genuinely hurt by ex girlfriend from the beginning.
The worst part is the author leaving an open ending where is hinted he will be with both of them however nothing is conclusive.
Stupid in so many levels it feels like a waste of time! And as far as I know in japan a marriage between 3 people is also illegal wtf lol
You've been warned.