Step 1: Preheat oven to 375°F (190°C) and grease a 9×13 inch baking dish.
Step 2: Cook macaroni according to package directions until al dente, then drain and set aside.
Step 3: In a large saucepan, melt 4 tbsp butter over medium heat, then whisk in flour to create a roux and cook for 2 minutes.
Step 4: Gradually whisk in milk until smooth, then cook for 5-7 minutes until thickened.
Step 5: Remove from heat and stir in cheddar, Gruyère, and Parmesan cheeses until melted and smooth.
Step 6: Season cheese sauce with salt, pepper, and paprika, then fold in cooked macaroni until well coated.
Step 7: Transfer macaroni mixture to prepared baking dish.
Step 8: In a small bowl, combine panko breadcrumbs with 2 tbsp melted butter, then sprinkle evenly over macaroni.
Step 9: Bake for 25-30 minutes until golden brown and bubbly.
Step 10: Let rest for 5 minutes before serving.