My daughter is also 3 and has never grown well. WE have been doing super high fat for a long time. One of the places I can really add a ton of calories that she always eats right up is eggs. I pretty well have our egg recipe mastered. One Large egg, 4 tablespoons Cream, 2 tablespoons either butter or coconut oil, 1-2 tablespoons pancake mix. The pancake mix holds the other stuff really well and it makes the egg nice and fluffy. Before I started adding it the eggs were really rubbery. I scramble it together really well and then stick it on the skillet, I flip it over like a pancake and then add 4 slices of cheese and fold to double. SHe loves it, eats every bite. It is about 700 calories, made this way. I also have a really high mac and cheese and pasta bake I make for her. She also really likes chili and I make that with a lot of cheese for her and no cheese for the rest of the family. Pasta with alfredo sauce is another thing she loves and then for the rest of the family we use red sauce. I also add chocolate syrup to her milk, it adds 100 calories. Chicken nuggets are a favorite in our house as is grilled cheese and you can butter both sides of the the bread. I have found that making the food fun works too, like cutting hot dogs to look like and octopus. Ramen noodles cook nicely in oil or butter and then soak up all the calories.