My statement was about how to not stand in line to book, paying full price and getting it taken care of before boarding. However if you are looking for a hack to the system then do this.
Go into the cruise planner pick your three restaurants , pick your days and times and pay full price. Go back into the cruise planner and purchase the three night package and pay the discount price. Once onboard go to guest services and let them know you purchased the three night package and would like to use that for your reservation keeping the same time and days. They will issue you a fully refundable OBC for the full price you paid. Many on cruise critic have done this in the past and we also have done it several times. We usually just pick one for a full price if there is a specific time and day we want. You can book cruise planner dinner reservations a year or more in advance.