To make possible to gather her for the Kaer Morhen battle, I need to succeed a romance. However, if I do it and I also romance with Yennifer (I want her for my love interest) am I doomed to this threesome scenario? Or can I choose again and decide on one of them again at the castle?
I got the achievement/trophy (playing on PS4) without even starting Triss' side-quest.
Here is what you need to do:
Don't start Triss' side-quest until you get the main quests that start with Brothers in Arms.
Then all you need to do is talk to Triss and ask her to come to Kaer Morhen.
- She will then say that you need to help her first before she can come along with you disregarding the romance choices you will make during her side-quest.
Provided you didn't kill Keira and did all the required quests for the others then you will get the Full Crew achievement.
If you already romanced Triss I'm pretty sure you have to choose her if you don't want the bad scenario, but I've heard there are a few ways to get her to the Fortress without romancing her; look at option 2 on this guide: http://www.ign.com/wikis/the-witcher-3-wild-hunt/Sex_and_Romance
I've also heard that you can let her go and she shows up anyways letting you get the trophy. Not sure on that though as I haven't beat the game.