Hi, Christine!
If you are planning on fun dining like character buffets and the Fantasmic! Dinner Package, the dining plan will worth it. A character buffet can cost (approximately) $35 or more, so just with one meal you are going to pay for a good part of your dining plan for that day. Add to that 1 quick service credit, 1 snack credit plus your resort refillable beverage mug and your plan becomes quite a good value.
While what you will spend out of pocket per day for food will vary based on your dining choices, we have kept track of our spending and for our family of 4 (2 adults and 2 kids) we usually are around $175 + per day. Now you only have 3 in your family, so that will be a bit less, but that should give you a very ballpark figure to work with.
One thing I really like about the dining plan is it is a great way to budget for you vacation. You may have a few extras you add on, plus gratuities, but overall you go into your vacation knowing that your room, tickets, and dining are all paid for. That makes going to those expensive dinners a bit more enjoyable when you know they are covered by the Disney Dining Plan.
If you need restaurant suggestions or have any other questions,
let me know! I would be happy to help you plan your magical vacation in Walt Disney World!