I was really hoping the other two games would release on the Switch before the third, I don't want to jump in blind...
First Witcher is PC only game (well, Mac is kinda PC also I guess), there was port of sorts in development way back for PS360, but it was never finished. Given that it's very mouse centric game, including combat, there were lot of changes, but deal fell apart in the end. As for TW2, I guess they could've done it, since there is 360 port of it.
But honestly, you don't need to play any of them, there are some references, but overall it's not that important. However, if you do want to play TW2, since TW3 is mostly open world version of it (personally I find TW2 to be a better game), maybe you can play it on PC given it will run on right about anything newer than 2009/10 (and some even older) and it goes for $20 on GOG and Steam.