Where can I find the guideof chapter 3 to buy
For now, the the extended guide for chapter3 stays for Vanir backers on patreon only. It will be added for sale here in the future.
You can join as Vanir backer on patreon to get access to it if you want to get it sooner (and also get access to chapter4 and the other extended guides), and ofc if you have specific questions about chapter3 you should join the discord server and ask there :)