Before coming to your answer, I want to tell you that CBSE class 12th board exam will be the easiest exam in your life. Now to score good marks in that you need to prefer right books.
Physics: For physics I will recommend you to read theory part from Pradeep or SL Arora as the NCERT is very boring of the first part. For the second part I will strongly recommend you to read to NCERT. Coming to question solving, solve every single example and questions from back exercise of NCERT. In the end when your course is complete buy any of the one either Arihant Chapterwise or Shiv Das as you will get to know how CBSE ask questions and how to answer those.
Chemistry: For this NCERT is enough to score 100 out of 100. Read NCERT line by line and solve every example, intext questions, and back exercises. And once your syllabus is finished start solving CBSE previous year question papers.
Mathematics: From the beginning solve NCERT. You can also solve RD Sharma but only those questions which are asked in CBSE. Revise NCERT at least 3 times before mid January. Once you are done buy NCERT Exempler and solve it. You can solve it in max to max 15 days