Bring a large pot of salted water to a boil. Add the macaroni and boil.
About 3 minutes before the macaroni is done according to the cooking time on the package, add the cauliflower.
About 1 minute before the macaroni is done, add the carrots. Continue boiling until the macaroni is al dente. Drain well. Transfer to a lightly greased casserole dish.
To make the cheese sauce, melt the butter over medium heat in a medium saucepan. Stir in the flour and dry mustard to form a smooth paste. Stir in the milk and bring to a boil, stirring to prevent lumps. When the sauce thickens, stir in the Cheddar until melted.
Stir the sauce into the macaroni and vegetables. Season with salt and ppper.
Bake for 30 minutes in preheated 350 degree oven until bubbly and browned. Serve hot.