How to make sure recent history has more weightage in forecast calculation?

We are considering 15 months of historical Bookings History in the demand plan. But we want the plan to generate forecast majorly (85%) based on recent 3 months of the bookings history and 15% to the rest of the months. What set up needs to be updated to make sure of this?

May be it's a really basic question for some of you, but I would really appreciate your response, thanks a lot in advance.