Alright time for an attempt at possible dialogue.......
Takes place at Vesemir's funeral....(Triss as the love interest of course)
Starts off with Geralt ready to light Vesemir's lifeless body on fire, Triss is standing next to him, she gently puts her hand on his shoulder and takes a step to the side (closer to him), Geralt quickly turns to her and looks sad (and so does Triss), whispering then occurs...
Triss: Geralt, I'm sorry about Vesemir
Option 1: We knew what we were in for
Option 2: It was all for nothing
Option 1 - Geralt: We all knew the consequences Triss
Triss: I know, if you want to talk about it.....
Geralt: Not now, please (Triss understands and retreats as Geralt lights Vesemir's body)
Option 2 - Geralt: It was all for nothing, Eredin will return
Triss: I know, but we can discuss that later
Geralt: Thanks
Dialogue ends there (most of the scene occurs with no words said), after when the player has control of Geralt and can talk to his allies, Triss stays back (or whoever was the love interest) and a chance to interact with her....
Geralt: Triss.....I am sorry too, i knew you were also close to him
Triss: Which is why we must make sure we protect Ciri and not let his death go for nothing...
Option 1: It won't Triss
Option 2: I will find them and make them pay (slight hint at him getting emotional)
Option 1: It won't Triss, nothing will happen to Ciri
Triss: I don't doubt you for a second,
Geralt: Triss.....thanks again, had me worried for a moment there
Triss: You had me worried too (awkward pause) ......come on Avallach is waiting for us (Triss heads off, player can choose to follow her or continue to interact with others)
Option 2: I will find them and make them pay
Triss: I want to see them pay too, but we all need to be careful and not to let a repeat of what happened here
Geralt: I know, it was close, you need to be careful too
Triss: Don't worry about me, i told you at the lighthouse that i would be by you and Cir's side
Geralt: I know...
Triss: Come on they are waiting for us (Triss proceeds, player choice to follow her or continue to talk to others)
That could be a nice add for Triss in TW3. But, imo, a conversation between Triss and Geralt before the battle in KM (maybe in Novigrad ?) would be useful to clarify things if Triss is the romance choice... Triss character would gain some depth during this process.