If you have booked through Disney and you have, or are planning to get, Magic your way tickets then it shouldn't be a problem. Otherwise you will need to speak to your TA
You would to buy only a base ticket for each member of your party so that's important if you already have tickets or are not planning on going to the parks all 3 days.
There is no minimum requirement for DDP as far as I'm aware, BUT you have to have it for every member of your party for the whole of your stay, and it has to be booked before you arrive!