Where are Yen and Triss?

Considering the story itself, I'm not sure about Yen whose theoretical location greatly depends on the ending but I believe that Triss departs to Kovir to become its king's advisor in any case, so it's logical she's nowhere to be found.

If Geralt romances Yen, they live together at an undisclosed location. If he doesn't and

Emhyr lives, she probably serves as his advisor and resides in Nilfgaard (remember that Philippa asked Geralt to take Yen with him because she wanted the position for herself).

Considering even as little as the above, the easiest way of dealing with those multiple outcomes while letting the player finish the secondary quests and contracts without adding new locations and conversations, is to let them simply disappear.

It is indeed unfortunate because one (or at least I personally) would expect that a couple of final conversations reflecting upon the whole adventure would be adequate and not too much considering the length of the whole story. I would appreciate at least a tiny location where I could meet either Triss or Yen, maybe Ciri

if she survived and became an almost-witcher,

and have one final conversation, much like in Mass Effect 2. Perhaps even do some contracts and then return for their witty remarks and some sexy time.

I believe that since I romanced Triss, that they should at least have points in time to wear you run into them. I think that Yen is up North helping the Nilffgardians and Ciri says to you during a mission, that she wanted to be just like the traveling elves. She said that MAYBE after all of The Wild Hunt business was taken care of, that she'd run off with them and not give a lick about anything. To just travel for the fun of it. But in the cut scene where it tells you what all goes down before you meet Ciri to give her the special ordered sword, it says that Geralt and Triss moved to a disclosed area in isolation and Triss became the advisor in Kovir and they lived happily ever after. So, i'm not sure if CD Projekt Red removed the characters for a reason or just if they weren't sure if a glitch were to occur to where it would put you back into the main story and screw up your game.