I was wondering which books should i take for N3 prep, cause i took a look inside Shin Kanzen Master N3 文法 but i am not convinced to continue with that book one of main reason is that some review sections give you grammar that you didn’t learn and then you learn it later or.
Also should i study grammar, vocab, reading and listening book at at the same time or should i learn it separate?
Hello 
Right now I am studying for N3 exam this July, at the moment I’m getting through Tobira, after I had gone through both the Minna No Nihongo. Its a really dense book full of grammar, kanji, readings and listening if you use their webpage. I can’t recommend it enough. Its also supposed to help you give you a bit of a base for N2.
As for other materials, I read NHK Easy Web News when I have the time. It’s a news letter written in N3 level Japanese, updated almost daily with 4 or 5 articles. Its pretty good for picking up vocabulary.
I’m also very interested in what materials other people are using, learning books and also outside of that too (Manga, novels, web pages, etc).
Before I did my N3 I often used https://japanesetest4you.com/
It’s got a lot of material and was especially helpful for me because there are lots of listening examples and my listening is my number one weakness. It has loads of actual test questions from previous tests. JLPT is all multiple choice so it makes it easy to study on this web site.
As for what to study, that absolutely depends on the person. It goes without saying that you should assess your strengths and weaknesses and focus on the weaknesses, but if you’re able to focus on more than one aspect of study at the same time, go for it! Just do a bit of everything every day.
PS are you in Japan? If so then try to only hang out with Japanese people. It’s incredible how much it helps. If not, find a load of friends on HelloTalk and text/call them every day.