I played all the games in the trilogy and read the books too, but Witcher 3 is completely worth it and much much better compared to the previous games in my opinion. You can buy it when it is on sale, play a bit and return it if you still think it is not your cup of tea. The beauty of this game is that a lot of the side quests are not fillers and almost every one of them has its own story that compliments the main quest line.
The Witcher 3 DLCs are also incredible. And if they release the mod support that will make it even better.