My own perspective regarding this is that it well could be true that the main choice outlined for you is Yennefer.
Though there may, this time, be a side choice for Triss, etc. [I say that, hopeful, based on older information from Red.]
As we all know, Shani was a choice by many, who was, more or less, tossed aside in TW2.
For those people, Triss felt 'forced' upon them.
In a way, Triss from TW2 had attributes that felt like they were more akin to Shani than to the original Triss character.
Agree or disagree with Shani as a choice... that being the point, there was a choice that was essentially stripped from the player.
Imagine getting adjusted to a new character, and then in TW3 the same thing being 'pulled' - "As much as we tried to 'force' Triss on you, its actually Yennefer!"
Personally, if that were the case, it feels a bit too much, in a scenario where your actions are supposed to matter -
I do understand that the game is based on the book series, where Yennerfer is Geralts lover.
However, not many people are aware of this... and to be honest, who is to say that after amnesia and gaining his memory back that Geralt is not a changed man based on the actions that took place on his journey back, as it were.
Think about Arnold in "Total Recall" - it could be true that he was an agent who purposely had his mind wiped... at the end of the day, he chose not to go back - true or not.
And that is what it feels this game needs... a true alternative.
Im pretty sure Shani is gone... but i hope that they leave options open, none-the-less.
p.s.
I read in a thread someone mentioning the possibility of Ciri.
While most people would immediately say, "No!" - it must be considered that in certain Central/Eastern European countries, that cousins are married to each other. [Im referring to after the war... not present day.]
And Geralt is not even a blood relative - also many people will not have been properly introduced to the character of Ciri.
This is not an attempt to convince anyone of anything, again it is user choice, but it is merely trying to highlight some points that may have been overlooked.
After all, this is Geralt - and Ciri is a grown woman, that is not blood related...
.... based on TW1, I would almost find it hard to believe the option would not be available.