Cook the macaroni according to package instructions. If you are using fresh broccoli, blanch in hot salted water for 5 minutes, then drain. If you are using frozen broccoli, thaw and drain it. Add the broccoli to the macaroni. Set aside.
Meanwhile, place the sun-dried tomatoes in a small bowl and add enough warm water to cover. Let stand until softened, about 10 minutes. Drain well, then chop into 1/4-inch pieces and set aside.
Melt the butter in a medium saucepan set over medium heat. Add the green onions and cook until tender, about 2 minutes. Stir in the flour, cayenne, basil and salt. Add the milk and cook, stirring, until slightly thickened and bubbly. Add 3/4 cup of the Cheddar, then all of the Gruyere, and Gouda, a little at a time, stirring well after each addition, until the cheeses are melted. Stir in the macaroni-broccoli mixture and the softened sun-dried tomatoes. Transfer to a 9-by-13-inch casserole dish and sprinkle the top of the casserole with the remaining 3/4 cup Cheddar. Bake until the cheese is melted and the casserole is heated through, about 25 minutes.