You do not need to get the multi time insinde the 3 years but you will need to pass the IR. Once you have the IR the exams are "frozen" and remain valid for up to 7 years after the expiry of your last IR currency check. In other words as long as you keep the IR current (or up to 7 years after it lapses) you can get the remaining requirements for the ATPL at any time in your future flying.
Grab a copy of LASORS for all the info.