Preheat oven to 350 degrees. In a large microwave-safe bowl, add broccoli, carrots, and cauliflower and stir to combine. Microwave uncovered on high until the vegetables are warm, about 12 to 15 minutes.
Transfer to a 9-inch by 13-inch baking dish. Evenly place the sliced Velveeta over the vegetables.
In a medium bowl, add Ritz crackers and melted butter. Toss to combine. Sprinkle over Velveeta.
Bake until cheese is melted and cracker topping is golden brown, about 30 to 45 minutes.
Vegetables: Package sizes may vary, so aim for about 48 ounces total (3 pounds, about 9 cups) of frozen broccoli, cauliflower, and carrots. To substitute fresh vegetables, blanch them in boiling water until tender, then drain and pat dry. You will need about 9 cups of chopped vegetables.
Velveeta: If you don't adore this melty, creamy, nostalgic processed cheese product, substitute homemade cheese sauce.