Maybe you are right. But even if Tremolo and Sage are in a serious relationship and lovey dovey with each other, as you say, Sage is wild enough to want to try a threesome. It seems that, sexually, she´s trying to make up with Tremolo for the time wasted with Chad. About Quinn, yes, she probably would not want to have sex with Sage and Tremolo if they are an "official" couple. But they are just about to have their first date. At the moment of the party, they could not be yet an "official" couple or just could not tell everybody yet so Quinn would not know they are "lovey dovey".
That party will has to have a really good climax to survive the hype is creating after more that a year waiting for it. If Tremolo goes to the party being Sage´s boyfriend, or at least dating with her, and can´t fool around with the HOTs and other girls, like he usually has done in other parties, is going to be a boring party. But, if DPC would have reserved that scene of the threesome for the party, imagine the story: Tremolo, Sage and Quinn have a few drinks, engage in some sexy games, and at some point, horny as hell, they decide to sneak out to Tremolo´s room and make the threesome. Even Riona or Lily, for example, could follow them and join... It could have resulted in an orgy!
I think a scene like that one would have more "weigh" than a threesome out of the blue in Quinn´s room. It would be a great climax for that party. And imagine the "walk of shame" the morning after, with the DIKs seeing all those gorgeous HOTs coming out Tremolo´s room, all of them green of envy