As others pointed out before, quite correctly, this is a choice that will in no way depend only on Geralt's desires.
First off, Yen, assuming she is her old self, isn't the kind of woman who just mopes around because some pretty redhead is hitting on her man. As far Yen is concerned, if anyone is going to be hitting on anyone, she'll be doing the hitting. She's had words with Triss about this in the past, and Triss eventually accepted it.
Second, in the same vein, Triss knows that her time is probably up. She managed to weave a half-lie to get Geralt to feel more strongly for her than he would have - and let's be honest, she lied to herself as much as she did to Geralt by deluding herself into thinking it could be - and by the end of TW2, she pretty clearly tells him that if he's going after Yen, then she's willing to help him find her.
Third and last: Ciri is not going to be indifferent to the outcome. Ciri sees Geralt and Yen as her surrogate parents and has gone to some pretty extreme lengths in the books to keep them (or even, at times, bring them back) together. She very much likes Triss, but I strongly doubt she'd be happy about it if Geralt let Yen go. As soon as Geralt finds her, she'll want to know where Yen is, and to see her.
In the end, I think CDPR can perfectly well end up not giving us any choice without it seeming hamfisted or odd for those who haven't read the books. They just need to establish that a) Yen won't have it, b) Ciri won't have it, and c) Triss loves Geralt/Yen/Ciri enough that she won't intrude if they end up reunited as a family again. Do that, and even players who never heard about the books will understand why they aren't being given any say in the matter.
I mean, if Geralt doesn't get a say in the matter, we as the players sure as heck don't either :comeatmebro: