Stating surprise at what Holiday Island does and does not do at this stage in its life is a bit nonsensical.
And I am not writing that to be impolite, but there are 237 pages on it so far and the game has been out on Patreon for quite some time.
So the information is out there if one wants it.
And the majority of its Patreons happen to like it for what it is. It is almost the only game out there, that isnt off on some niche fetich.
Anyways,
Going out on a wild guess, I would blindly speculate that three-some will make it into the game.
I dont have anything to back that one up, besides that Amy-Alice seems a pretty obvious bet on it and that a number of the girls could probably go for it in terms of sexual drive, curiosity or desire etc.
But since the current efforts is to bulk out and provide the foundations for the many girls, some of which still have little, my guess is that three-some is more of an end game occurrence.
That is to say, something that will story/relation-wise only be able to occur further into the game, and thus is further down the pipeline than what we can see.