The DxDP is 3 meal credits, two snacks per person per day. Meals can be TS or quick service. When at a TS, it includes an appetizer, meal, dessert, and drink per credit. QS is still just a meal and drink.
If you were to put the DxDP on your 3 person room, you'd pay $348.75 a day and get 9 meal credits and 6 snacks a day, plus 3 refillable mugs for the length of your trip. You could then use those 9 credits to do 2 TS meals (assuming you're willing to split 4 apps, meals, desserts, and drinks among the 5 of you at an a la carte TS, which is still a ton of food). Loo at Disney menu prices and you'll see that it's nearly impossible to feed 5 adults 2 TS meals at that price if you're paying OOP.
Really, look at how many meals you want to do sit down and then decide of you want the plan on the 2 person room or the three person room.