Is it true that all 3 papers will include research methods?
Essentially, yes. Paper 2 has its own section on reseach methods but it can - and certainly will - come up in every paper, even likely in ever topic. According to AQA (as stated in the inside cover of the textbook), "A minimum of 25% of the overall assessment will assess skills in relation to research methods. A minimum of 10% of the overall assessment will assess mathematical skills for both AS and A2." Also, bear in mind that each topic is worth 24 marks but the research methods section is worth 48.