I think theyre all just kind of individually broken kids holding each other up as a support system. It kind of slips past because this manga has been presented in a standard romcom setting but these three(and Ashaina to a lesser extent) have all been fucked formatively.I still dont get yui and seiya's relationship at all. Some chapters it seems she likes him, but others they just seem to be "comrades in arms", if you get my meaning, its like her feelings change by the minute.
Yui probably definitely does romantically like Seiya, and I'm not sure if Seiya actually has any romantic interest in Asahina, but I doubt Yui really cares about that either way. Their living state has most likely made her completely ambivalent to the concept of monogamy, and she more seems distressed by the fact that Seiya wont be around any more, rather than the fact that he would be dating someone else.