Hi Maria!
You're definitely onto something! This is what our family does when we get the regular dining plan:
We're not big breakfast eaters, so we can generally get away with a snack such as a fruit or cereal in the morning, and then choose either table service or quick service for lunch and dinner. You'll find Disney provides very hearty meals, so a big breakfast (especially if it's buffet) will almost carry you through lunch, to where you just need a snack, and then in the evening, get a quick service or table service to finish the day.
Unless it's a buffet, quick service and table service meals are both large meals. The only difference is being hosted, as is the case with table service.
Unless you're a sit-down-at-each-meal family, I think you may be able to get through using the plan, and little of your own cash...
If you do feel the 3-meal approach is best, then you're almost approaching the Deluxe Dining Plan option. Although the upfront cost seems expensive, you'll end up saving more than paying out of pocket in the long run.
But if you had to, then you're right in that breakfast should come out to be a little less than paying cash for dinner...
Have a terrific time! The food at Disney is excellent!