Well, a Threesome is, at its base, just an accord to have sex between three people, but how kinky it gets depends solely on the implicated parts. It could just be that they take turns, or it could imply doing it the three at the same time (without contact between the same-sex partners), or it could also be about them all having a small orgy together (implying bisexuality and incest in this case, which I agree would be kinky but I'm not sure if "too kinky" considering what other stuff there already is in the game). You can even go the extra length and have a Threesome combined with BDSM elements, the level of kinkiness is very variable and I'm sure the devs could find one that they feel comfortable with.
That being said, I also agree that, while the idea of a Threesome is very appealing for a majority of people, it could very well not lead to it and instead just lead to a hook up, a "lending" so to speak (or "sharing", if the former word sounds too kinky), something which we already have in the game so it wouldn't be too unrealistic.
BUT, just to be clear about something, I don't think this devs are planning to keep game purely vanilla forever. In their message, it seemed more like they were open and interested in knowing what our kinks are, so don't lose hope my friend.