I don't think it will be a choice.
Just like in Ep 11 the player gets a solo date with either Maya or Josy depending on which they have done more solo things with, I suspect the game will use that action to drive a wedge between the two that had the date and the 3rd that is left out.
I am confident the player will still be able to salvage the throuple but it will take some time to rebuild the relationship between all 3.
And in that process the player will instead be able to just pick a solo with Maya or Josy route if they want. (Though it would take a real heartless b*stard to kick either Maya or Josy out of the relationship and just keep the other.)
I suspect the wedge will happen in Ep 12 and the rebuilding and choice between throuple or solo maya or solo josy will be in Season 4.
I have a very different and slightly more optimistic outlook and 2 possible ways I can see this going.
First of all, this is the "Josy
and Maya branch" and I don't think that it would be a branch for and with both if you can't end up with both. This game is not the same one as AL and I don't believe it can be used as a precedent because the throuple in that game never even got started because the moment you made it official you wanted that, which in this game would be the equivalent of the choice in Ep 4, tragedy struck soon after and it ended before it even began. With this one, we entered into it fully and have been doing our best to navigate its complexities whilst also getting intimate time with both at the same time, something AL did also not have.
With that being said, I think that what we will first and foremost see is that Josy & Maya will follow through on what they said talked about in Ep 11, opening up more and spending more time together as they look to rekindle things between them which they both said they wanted. We'll also see them and the MC doing things together as part of this as well since they also acknowledged they shouldn't leave him out of this process.
After this, we'll get a scene where the girls want to have a serious talk about how things have been going, maybe at the NYE party, and the first way I could see it go is that it will come down to choices made throughout the game that will feed into a scoring system like what Zoey got in Ep 11. If you score below the threshold then the MC will be the one who exits the relationship because it will be such that they feel as though his heart isn't in it with both of them and he will realise they're right, and if you're above the threshold then he will all say they love each other and the throuple is set.
Another way I could see it going is that a scoring system adds up things with each girl individually, not just the lewds but other choices as well, and if it's more heavily weighted towards one girl than the other then it either results in the other acknowledging that they think it best if they step aside and let the MC and the girl with the higher score be together, or it results in both girls feeling the same as the previous scenario, that his heart wasn't in it for both and since this was the whole point of what they entered into they feel they have to let him go.
A possible 3rd way it goes is that this simply gets dragged out until S4 because Ep 12 is the rebuilding episode and will be all about them recommitting to this after all the drama that's just happened kind of threw them off track and they weren't as focused on it as they should have been. So it won't be until the last season and maybe even near the end of the game before we see how it goes.