Seems like both Heejae and Sunyool are already falling in love with Jisuh, but does it necessarily mean a "happy ending" if (how the things are now) Jisuh is more than interested in Heejaem and Heejae completely unwilling to let Sunyool closed? If the tendency doesn't change, Sunyool is going to be letf outside, even if he fall in love with Jisug completely, but my guess it this is not what you meant when you wrote about "a happy ending", or is it?
*to let Sunyool close* of course, what a silly typo.
Besides, if Heejae and Sunyool don't melt for one another, it'll still be a love triangle instead of a threesome, and an edgy one at that.
I hope this ends on a threesome with Sunyool and Heejae both in love with Jisuh. I would be happy with that ending. Either that or SunyoolxJisuh!