1. Heat oven to 350 F. Coat a 2-quart broiler-safe oval baking dish with nonstick cooking spray. Bring a large pot of lightly salted water to boiling.
2. Once water boils, add penne. Cook 6 minutes, then drain. In a large bowl, toss together the cheddar, Gruyere and Asiago. Set aside.
3. Meanwhile, melt butter in medium-size saucepan over medium heat. Whisk in flour until smooth and slightly bubbly. In a thin stream, whisk in milk. Stir in onion powder, salt and cayenne. bring to a boil over medium-high heat. Reduce heat and simmer 3 minutes. Remove from heat; whisk in 2 1/2 cups of the cheese mixture.
4. In pasta pot, stir together the cooked penne and the cheese sauce. Pour half into prepared baking dish. Sprinkle with a generous cup of the cheese. Spoon remaining penne into dish and top with remaining cheese.
5. Bake at 350 F for 20 minutes. Increase oven temperature to broil and cook 3 minutes, until top is lightly browned. Cool slightly before serving.