*Spoilers, obvi*

In the base game the two main love interests are Yen and Triss. There are some interesting interactions at play so I'll refrain from spoiling that as much as possible. There are a few other characters (other than women at brothels) you can romance that have small side stories.

Hearts of Stone brings Shani back as a love interest, but does not affect your main game.

Blood and Wine has a purely sexual encounter with a character I will not name for slight spoilers reasons within that story. This expansion also takes into consideration your romantic choices in the base game.

If you want more specific information just let me know. Kept it vague to avoid more spoilers.

Of love interests, there are only two: Yennefer and Triss. All of the other 'romantic' encounters, including Shani and Keira, are what you might call 'one night stands', at least as far as the romance/sex is concerned.

Yennefer is the clear written favourite (books and game), born out in the game by the lack of dialog content for Triss in this matter. CDP said this issue, (users' TW2 decisions romance-wise) was going to be addressed, but it never has been.

Thanks to both of you for the info and letting me know! I'm disappointed it seems so heavily skewed towards one person (Yen), especially since I prefer Triss, but I'm ecstatic to know Shani is back (she was my favorite in the original), even if only for a brief reappearance. I actually wonder why Shani was written out of the games the way she was... maybe the developers didn't like her much in hindsight? Or maybe she had little to do with the source material (mind you, I haven't read the books yet).

Well, I'm glad there are options at least, as I love variety in games. Hopefully CDPR throws in some kind of bonus for Triss then... I'm sure there are plenty of other Triss fans who would appreciate it as well!

^ This. It's the same reason Triss got shouldered out in favour of Yen. Yennefer is Geralt's one and only 'true love' in the books.

I thought some of the later patches did add in some dialogue and scenes for Triss due to complaints about how she vanished after the completion of her quest line. I think the scene right before the final battle is new. I thought there were some additional lines or scenes added at Kaer Morhen, too. Can anyone confirm?

I do wish there had been a bit more between the two, but I was shipping Triss & Geralt since the first Witcher game (I'd never read the books, so wasn't at all familiar with Yennifer). On the whole, though, I was satisfied with their story, and thought it worked. However, I haven't done a Yen/Geralt playthrough, so my opinion might change if they have more interesting dialogue and scenes together.

Edit: I believe Blood & Wine adds an extra scene for both romances, depending on who and if Geralt chooses in the main game. I haven't seen it yet. However, an early quest in B&W starts with a letter from Triss if she and Geralt are a couple, and from Yen if she and Geralt are a couple (or, I assume, if Geralt is not actively romancing either of them). I thought that was a nice touch.

Her role has not been addressed above a few added comments. She still has the "Well?" reply to any attempt at conversation outside of quests. But to be honest, since I can't stand the sight nor sound of her, I haven't pursued her romance to notice any difference.

SPOILERS

I think there were two added dialogues related to Triss romance added in patches. One is Ciri asking Geralt about his thoughts on why he chose Triss over Yen. Another is in Kaer Morhen after Ciri is found and everyone reunites. Yen offers to give Geralt and Triss some time alone - but interrupts before they can say much.

The game favors Yennefer. I imported my Triss-loving Geralt savegames from TW1 and TW2 and even I ended up romancing Yen in my first "gut-feeling" playthrough. Everything about it just makes sense, from the final breakup with Triss, to Yennefer breaking the spell that "bound" her and Geralt, to the two finally just acting on their feelings knowing there's no magic destiny at play, it just all fit together.

I've romanced Triss in my latest playthrough and it hits a lot of wrong notes. I guess I know how Shani fans felt after she was pushed aside in TW2.

Keira Metz, Shani, and the Iron Maiden are non-exclusive romance options, but you also have the chance to say 'there's someone else" or similar lines. There's also the prostitutes.