Preheat oven to 350°F. If baking lightly butter a rectangular 8" x 12" baking dish. Cook pasta, per box instruction, to slightly less than al dente for baked mac and cheese, or al dente for stove top version.Drain and hold. Melt butter in a large pan over medium-high heat. Whisk in flour and paprika, and stir constantly until slightly thickened, about 1 minute.
Gradually whisk in milk and cook until thickened, about 4-5 minutes, stirring continuously. Mixture should be thick and smooth.
Turn off the heat.
Blend in the Dijon mustard.
Stir in 1 ¾ cup cheddar cheese, Parmesan, and Gruyere. Mix until melted and smooth.
Stir in cooked pasta, mix to coat and season with salt and pepper, to taste.
If baking spread mixture into a prepared baking dish. If serving as stovetop mac and cheese simply serve, as is. If baking, combine breadcrumbs or panko with 1 tablespoon of melted butter. Sprinkle mac and cheese with remaining cheddar cheese and prepared bread crumbs or panko.
Cook for about 25 minutes, or until bubbly, and bread crumbs look toasted. If bread crumbs start to get too dark, cover the pan with foil.
Let rest for about 10 minutes before serving.