It may sound very like a very old thing that almost everybody says but I m gonna restate it and say that you will need a lot of practice to get some decent marks in P3.
Its very important that you have a decent idea about the models and then you go on attempting the revision kit questions (only the actual past exam questions). Near to the exam you can attempt around 5 to 6 recent exam papers and you are good to go. You actually need to build up on your writing style and the way you approach the question because thats the actual thing being tested in this paper.
For F8 a lot of rote learning will be required so the best way is to get the grip of all the concepts and learn points from all areas then go on to the past papers and try attempting as many questions from the kit as possible. Read the question and formulate an answer in your mind or jolt down the points that you think are applicable then go back to the actual answer and evaluate yourself, also get an idea of how the model answers are written and framed. and as a general point do pay attention to the way you present your answer, that makes a whole lot of a difference.