First off I guess I need to explain that I am not a Book reader, I was introduced to the Witcher universe through The Witcher 2 so arguably I'm heavily influenced by Triss' character in that game coming into The Witcher 3.
Even though I choose Triss I am very interested in Yennefer and everything that she brings to the series, my interest in Yennefer was created by her being mentioned as being important to Geralt while also seeming to be a touchy subject with Triss during Witcher 2. So even though I choose triss I was excited to meet her in Witcher 3.
After playing through the game I saw Yennefer as being more of an old flame, someone who was and still is very important to Geralt and while I felt that they truly loved each other deeply I also felt that they were not "in love" with each other at the same time.
On the other hand there is Triss who I see as ending up falling deeply in love with Geralt during Yennefer's absence, probably not intentionally but stuff like this happens in the real world all the time and it happened here in the game series.
You have Geralt (a known womanizer) with amnesia whose significant other (yennefer) is gone, though he still feels the pull of the Djinn's magic.
You have Triss a somewhat naive young sorceress who wears her heart on her sleeve now spending all this time at Kaer Morhen with Geralt.
I see Triss succumbing to Geralt's probable advances during this time and eventually falling for him HARD, and I see Geralt who may have initially thought of Triss as just another sexual conquest falling for her not only because of her undeniably charming personality but also because of the way she is with Ciri. All of this occurs even with the Djinn's magic still in effect.
I got the impression in the witcher 2 that Triss was not proud of the fact that she had fallen in love with her friend's man, but shit happens and she found herself in a very tough situation possibly setting herself up for a very hard fall if Yennefer ever returned. Despite this Triss decides to help Geralt recover his memory by recovering a rose of remembrance because she feels his struggles to regain his memory.
In the witcher 3 we find Geralt alone on Yennefer's trail I believe he was without Triss because she decided for Geralt's best interests to remove herself from the situation so he could figure out his feelings for Yennefer without any distractions from her, as hard as this must have been for her to do she was preparing for her hard fall.
So I see Geralt falling in love with Triss despite a Djinns magic, and its plain to see how deep in love with Geralt Triss is, I see Yennefer as being able to handle this as she already knew that Geralt and Triss were together so she stayed away, I think she stayed away because she knew that even though she loves Geralt she was not "in love" with Geralt anymore and this is why she pushes Geralt to help her find the Djinn to remove the magical bond once and for all.
To summarize, Geralt and Triss ended up falling in love during Yennefers absence, it happens. I don’t see Triss to blame for this and knowing Geralt as well as she does I dont think Yennefer blames Triss either.
Anyways those are my NON BOOK reasons for choosing Triss, I think Yennefer would be fine while I think Triss would be utterly crushed if Geralt didn’t choose her and I see Geralt realizing that while he does love Yennefer he is in love with Triss.