Sorry guys but I really don't know how can I download The Witcher 3 with GOG Galaxy because I have the button "Install" that download 33 GB, but in the same time in MORE--> GET BACKUP AND EXTRAS there are 3 files: 1 The Witcher 3 1.06 (22 GB); 2 Patch 1.07 part 1 (1 MB); 3 Patch 1.07 part 2 (2.2 GB).
So I should click the normal "Install" button or I should download the 3 files in in the section "GET BACKUP AND EXTRAS" ?

Just click install. You can use the other files too. They are just backup files. You could move them on an external hard drive so you don't have to redownload all if you want to play the game on an other PC (for example).

Yes I tried to download from install button yesterday but after hours it arrived at 100% but it stuck on "installing". I restarted gog galaxy and the download restart from 86% ...

My advice would be this:
Move the 3 files you found under "GET BACKUP AND EXTRAS" to a diffrent location on your PC. Deinstall GOG Galaxy. Run the install from this file: The Witcher 3 1.06 and install the patch after this. Don't use Galaxy for now. It is called a Beta but Alpha would descripe it better.
Make sure you have enough space on drive C (also if you install to another drive). Reason: The windows temp folder is on drive C.

So first of all I should download this 3 files and after I should go in the folder where they were downloaded and move them in another folder that I want ?

If they aren't already downloaded: yes. Or you could use your browser and download the game (7 parts) and patch (2 parts) from your library from the GOG website.
My advice with moving the files away was given because I would suggest to deinstall Galaxy and I don't know if all files get deleted if you do so. So better move them away.

Or you could try to use the file Galaxy has already downloaded. But I don't know the location (somewhere in the Galaxy folder I would guess). You could try to find it with the windows search.

Ok finally I have installed the game and the patch without problems! But now how can I install the free dlc that I have found in the store ?

Here is a guide: http://www.gog.com/forum/the_witcher_3_wild_hunt/guide_free_dlcs_activasion/post1