You can get the dining plan but you will have to purchase it for all three of you for the full length of the stay. After your second daughter leaves, you can use the extra credits for more meals for yourselves. Credits can be used by anyone registered on the reservation. I am going to assume that the daughter is an adult (meaning she is at least 10 years old). If you have anyone under 10 on the reservation, they'll have child credits, which can only be used for children's menu meals.
You will need to take note that the daughter who is there for the first three nights will have to buy the same park tickets you and the other daughter are buying, in the same amount of days, even if she is not staying for the entire time (if you are booking a package reservation - on a package, everyone registered must get the same dining plan and tickets)