You've got a few options.
1) Find the cheapest rate you can for three nights to put you over the top
2) Don't worry about it and hope Marriott extends your Platinum status another year since you are close (this does sometimes happen).
3) Wait for the "Buy your status back" offer in early 2015. It will cost 40,000 rewards points to remain Platinum. Look for such an offer in late January. You'll have until April (I think) to decide.
Worst case, you do nothing and end up Gold in 2015.
bejacob makes 3 good suggestions, but here is one more.
Call the platinum support line at the beginning of January and ask if they can extend your platinum status. Twice I have done this and been given the platinum status for the next year. Once I called and explained that I had been ill and had been forced to cancel a two week trip in December that would have put me over the required 75. The other time, I was 1 or 2 nights short and asked if they could help me. Both times the person accommodated me. The support line personnel always try to be helpful. They are a great benefit provided by Marriott.
Marriott is clear that "If a member downgrades to a lower status (e.g., Plat to Gold, Gold to Silver) there would be no rollover nights"
Rollover nights are a great thing, but the rules can easily trip us up. I learned the hard way a few years back.
If for some reason your budget or your schedule does not permit the three stays, call Marriott Rewards before year end, explain your situation and ask them if it's possible to qualify you as Platinum for 2015 because you are so close. I know that it has worked for others so it's worth a shot, but I would make sure to call them before 2014 ends.