It depends upon the choices you make which determine Geralt's motivation. At the Elven Bathhouse Triss ask Geralt to walk away from the whole Kingslayer thing for fear he'll get caught up in something too big to handle and just be with her. Triss also asks Geralt whether he'd prefer to travel alone in search of Yennefer,or whether he'd like her company. You can choose yes or no to that with the latter resulting effectively in a break up. Once Triss gets kidnapped, if you chose Iorveth (I can't remember the Roche path) you can choose to allow Stennis to be hanged, in which case you'll get the blood needed to lift the curse of the battlefield, or you can cross it in search of Triss. Finally, in Loc Muinne, you can choose to search for Saskia or Triss.
Now if you choose Triss in all these options, as I did, the story is framed in a way which says that Geralt couldn't give a Monkey's Uncle about the Kingslayer and is only in search of rescuing Triss. So you can play the game in two ways - either finding the Kingslayer and clearing your name is important or finding Triss and rescuing her is important.
You can read how I played it here:
http://forums.cdprojektred.com/threads/51724-Triss-Merigold-(all-spoilers)?p=1832058#post1832058
I highly recommend people who want the Triss romance fixed play TW2 again and play it in a similar manner as I did because you'll see just how big the divergence in her story has become in TW3. It's as if none of the events of that game ever happened.