First of all, it´s great you are so forward-looking. Haivng 3 months to prep is an absolute luxury! :) It will make you enjoy the way.
You have 2 main workstreams ahead:
1. FIT INTERVIEW
The "Integrated FIT guide for MBB" has been recently published in PrepLounge´s shop (https://www.preplounge.com/en/shop/tests-2/integrated-fit-guide-for-mbb-34)
It provides an end-to-end preparation for all three MBB interviews, tackling each firms particularities and combining key concepts review and a hands-on methodology. Following the book, the candidate will prepare his/her stories by practicing with over 50 real questions and leveraging special frameworks and worksheets that guide step-by-step, developed by the author and her experience as a Master in Management professor and coach. Finally, as further guidance, the guide encompasses over 20 examples from real candidates.
You can see plenty of reviews from candidates who purchased it already.
Furthermore, you can find 2 free cases in the PrepL case regarding FIT preparation:
Intro and CV questions > https://www.preplounge.com/en/management-consulting-cases/fit-interview/intermediate/introduction-and-cv-questions-fit-interview-preparation-200
Motivational questions > https://www.preplounge.com/en/management-consulting-cases/fit-interview/intermediate/motivational-questions-fit-interview-preparation-201
Behavioural questions (ENTREPRENEURIAL DRIVE) >https://www.preplounge.com/en/management-consulting-cases/fit-interview/intermediate/behavioral-questions-entrepreneurial-drive-fit-interview-preparation-211
Feel free to PM me for disccount codes, since we still have some left from the launch! :)
2. BUSINESS CASE
Given that you have the time, you can dedicate a couple hours to to the classics of Viktor Cheng, "Case in point" book, etc. However, you also need to get hands on asap:
1. Practice cases with partners asap, as many as you can do.Find experienced partners who can provide a good feedback
2. Practice your math skills, both in your cases and with ad hoc exercisess, such as:
1. Multiplying double digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo
2. Leveraging math tools (Mimir math for iOS), Math tool on Viktor Cheng website to practice
1. Profitability cases- basic profitability framework.
2. Idea generation cases: for any specif issue
3. Growth cases: market penetration, new product launch, product mix change, etc.
4. Pricing cases
5. M&A cases
6. Valuation cases
7. Value chain cases
Given you are under this much time contraint, consider working with a coach to foster speed and ensure you have a tailored plan to be very targeted. PM me interested!
Hope it helps!