01 - Preheat oven to 350°F. Generously butter a 2-quart baking dish and set aside.
02 - Bring a large pot of salted water to a boil. Cook macaroni until just al dente, approximately 1-2 minutes less than package directions. Drain thoroughly and set aside.
03 - In a large saucepan over medium heat, melt 3 tbsp butter. Whisk in flour and cook for 1-2 minutes until lightly golden, stirring constantly.
04 - Gradually add milk while whisking continuously to prevent lumps. Continue cooking until sauce thickens, approximately 3-5 minutes.
05 - Remove from heat. Stir in cheddar, Gruyère, and mozzarella until completely melted and smooth. Add Dijon mustard, garlic powder, onion powder, paprika, salt, and pepper. Taste and adjust seasoning as needed.
06 - Add drained macaroni to the cheese sauce, stirring gently to coat evenly. Transfer mixture to the prepared baking dish, spreading into an even layer.
07 - In a small bowl, combine panko breadcrumbs with melted butter and parmesan cheese. Sprinkle mixture evenly over the macaroni.
08 - Bake uncovered for 20-25 minutes until the top is golden brown and the sauce is bubbling. For extra crispiness, broil for 2-3 minutes, watching carefully to prevent burning.
09 - Let stand for 5 minutes to allow sauce to set. Garnish with fresh parsley if desired, then serve warm.