Welcome Lesta. Always nice to see new people joining.
To
fully understand there's no way around it. Not only are the two previous games required for a complete comprehension of the plot and the characters, but so are the preceding books.
That being said, CDPR repeatedly stated that TW3 is a standalone game and that they're keeping in mind that they may be newcomers to the franchise they'll need to bring up to speed. A common speculation in the community is that they'll probably use the tutorial section not just to introduce players to the mechanics but also newcomers to the story and the background.
TW1 is available only on PC. TW2 is available on PC as well as XBox 360. CDPR don't plan on transferring them to the new consoles. After TW3 they're turning their attention to their next new game, Cyberpunk 2077.
(Also, if you plan on purchasing the games on PC, I recommend doing it via GOG.com rather than steam, since GOG and CDPR are connected and purchasing from that site aids CDPR more - just if you want to give them a little nod as a fan)
Hope this clarifies
And with that out of the way, I'll take the opportunity to highly recommend both the books and the previous games. The secondary reason is because having such a vast background understanding will probably make TW3 all the more powerful to you. As an anecdote, I read the books after playing the first game and falling in love with the franchise. After finishing them, just the title of TW3 ("Wild Hunt") made me look at it in a completely different and more profound way. After the books I understood just how much weight stands behind those two words alone, while before I just thought of them as some random fantasy trope.
I then replayed the first game, after the books, and while the first playthrough was still amazing, in the second I felt significantly more connected. It's not that I didn't understand the plot when I played before the books, but playing it after reading makes you catch all sorts of nuances that you have no way of noticing before, which make the experience much more enjoyable.
Honestly, how many games in the industry enjoy such a deep and quality background? It's a shame to miss it, for anyone who's dedicated to making the most out of stories.
I don't want to overwhelm, of course! This shouldn't be a chore and you know best how much time you can, or want, to dedicate. That's why reading the books and playing the games for a richer TW3 experience is just a secondary reason. The primary reason is, well... because they're both awesome. Really. I started reading the books for the sake of the game and very early on found myself enraptured regardless of the games. And the games themselves are also stunning, with unique atmosphere and meaningful consequences to difficult decisions that very few RPGs in particular and games in general manage to mimic. Not to mention the lack of black-or-white dilemmas that make you heavily consider each decision.
http://forums.cdprojektred.com/threads/16494-Our-Community-Fan-Translations
That thread in our Community section has fan translations of the few books which aren't yet translated to English. I'll stop at this point because I feel I'm being a bit too preachy, but if your curiosity is piqued we'll be happy to give you more information and some guidance about the order to tackle each piece in this franchise