I actually haven’t played the other two since they’re both very old games at this point but I do plan to try them out. Witcher 3 seems like it mostly stands on its own and you won’t be missing anything too huge when it comes to the story and characters from the other games. That being said, its a situation where knowing the prior games helps and you’ll understand the relationships better. If you bounced off of the first one though, my recommendation is just to at least read up on the characters and major events of the first two. Maybe watch a game movie if you can find one. I went in completely blind into the third game and it turned out fine so really you can’t go wrong, don’t let the prep for playing the game stop you from actually playing it.