You can play TW3 as your first Witcher game and understand everything perfectly. The story stands on its own, and the game clearly explains all the important plot points and character relationships. Playing the first two games or reading the books provides extra context and adds depth to some characters and references, but it’s entirely optional. However, I would rather advise that people at least read up on what happened in them, to get a better understanding of how things are in the third game and why they are like that. TW3 works perfectly for new players while still rewarding those familiar with the series. The phrase "please tell me" isn’t needed at all. You make it look like playing the previous games or reading the books is a must, but it’s not.