I didn't follow the devs interviews or any spoilery article. I just watched the trailers on youtube once they were released. So there might be things I might have missed but my best guess would be by the end of the witcher 2 they already had the script of Witcher 3 and everything was set in the stone and maybe they didn't expect Triss to become this popular. I realize that many romanced Triss in the first game but not so much to consider as a real rival to the main character as Yen so at that point an option to choose Triss over Yen wasn't on the table but short after Witcher 2 they decided to revise the script and quests. If I'm not mistaken the first Witcher 3 gameplay footage came in 2013 and Witcher 2 was released in 2011 and I know there are a lot work dedicated to witcher 2 between 2011 and 2013 as I know there is an Enchanted Version that I bought. I think revising the script in such periods between two games would cause such inconsistencies regardless of how much effort they put in it.
I just don't think that's a valid excuse. There was a big stink about the Shani treatment in TW2. They would have also been fully aware of the popularity of the Triss character, which they themselves fuelled through secondary media. So sorry but a pretense of being caught off guard by her popularity just doesn't hold water in my eyes. Unless we get a dev statement confirming that they really originally planned to only have Yen then i won't believe it as it would mean the developer dropping huge amounts in my estimation.