Bring a large pot of salted water to a rolling boil. Add macaroni and cook until just al dente according to package directions (about 1–2 minutes less than package time). Drain and set aside. Tip: Reserve a cup of pasta water if you want to loosen the sauce later instead of adding extra milk.

Heat 1 Tbsp olive oil or butter in a large skillet over medium heat. Add shrimp and scallops, cooking about 2–3 minutes per side until just opaque. Stir in lump crab meat gently to warm through. Remove from heat and set aside.

In the same or a clean saucepan, melt 4 Tbsp butter over medium heat. Whisk in 1/4 cup flour and cook 1–2 minutes to form a roux. Slowly whisk in the warmed milk and the heavy cream until smooth. Add Dijon mustard, Old Bay seasoning, garlic powder, salt, and black pepper. Simmer gently, stirring often, until the sauce slightly thickens (about 5 minutes). Remove from heat and stir in cheddar, Gruyère, and Parmesan until melted and silky.

Add the drained pasta and cooked seafood into the cheese sauce pot. Stir gently until everything is evenly coated. Taste and adjust seasoning. For a crunchy top, preheat oven to 375°F (190°C), transfer to a buttered baking dish, top with panko mixture and bake uncovered for 20 minutes.