It hasn't reached that stage yet, actually. Up until the latest raw chapter, we know that Jooin is in love with Yahwi only. So far, the only role Cain has is to make Yahwi realize his feelings for Jooin and act on it appropriately. And from what I can see, Jooin's sex scenes until the end will be with Yahwi only.
If the author plans to prolong the manhwa then there's a chance for Cain but if they're setting up it to end at let's say less than 100 chapters, then Yahwi will be Jooin's love interest only. Either way, the two of them are endgame. Although, I don't really like Yahwi as a character for many reasons but that's besides the point.
Keep in mind that up until Cain's confession, Jooin has only ever been in love with Yahwi only. We can only guess if Jooin will really think about Cain's confession. But if you ask me, I think he'll turn him down.
someone asked author in twitter who will be endgame.she said i didnt decide yet
Many didn't really think it was because Cain at first had no romantic interest in Jooin until recently, he didn't even know he had feelings until now
I'm confused as hell y'all saying that this is not a love triangle but, the latest chapter cain confessed his feelings for jooin and he kissed jooin hands (omfg I ain't gon lie this shit is so romantic amd sexc too) and jooin said that he will think about it, and cain replied that he is willing to wait even it takes forever or sum so what the actual fuck