I agree, I hope they do a good job developing Yen's character in TW3 and providing more of a chance to find out what Triss is really up to.
I played both games first before I read the books. Triss made me uneasy in TW1 because I felt like she was hiding something important from me and I didn't trust her after the mirror scene. Then in TW2 when I was stuck with her anyway I tried to give her the benefit of the doubt, but as the game went on I started hating her for lying to me and not trusting me with the truth.
If she had just told me what she suspected instead of dragging Cedric into it, then I think that would have prevented a lot of problems that I had to clean up. By the time it got to the end of the game on my first play-through, I didn't even hesitate to save Anais instead of Triss. I was relieved that Letho saved Triss anyway because I don't hate her enough to want her dead, but even if she had died I wouldn't have regretted saving Anais instead (or Saskia for that matter).
Then I started reading the books and Triss is even worse than I thought she was