Preheat the oven to 375°F (190°C).
In a 10-inch ovenproof skillet, melt 2 tablespoons of the butter over medium heat and add the breadcrumbs. Toast the crumbs in the butter, stirring, until they’re a light golden brown. Season with a pinch of salt and scrape crumbs into a dish to set aside.
Place uncooked pasta in a large bowl and cover with hot tap water. Soak for 10 minutes then drain it.
Combine the three cheeses on a board.
Return the skillet to the stove over medium-high heat and melt the remaining 3 tablespoons of butter. Add the flour, whisking to combine. Add milk, ½ cup at a time, whisking until smooth. Season with salt, black pepper, cayenne, and nutmeg, and cook until mixture thickens.
Add the reserved cheese mixture to the sauce, stirring until melted. Stir in drained pasta until evenly coated. Sprinkle with reserved cheese and toasted breadcrumbs.
Transfer the skillet to the oven and bake, uncovered, for 30 minutes. If needed, add another 5 minutes for it to soften. For a browned top, run under the broiler for a minute.
Let rest for 2 to 3 minutes, then serve.
