Cook Pasta: Bring a large pot of salted water to boil. Cook jumbo shells according to package directions until al dente. Drain and set aside.
Sauté Vegetables: Heat olive oil in a large skillet over medium-high heat. Add onion and cook 3 minutes. Add bell pepper and cook another 3 minutes until softened.
Cook Meat: Add ground beef to skillet and cook until browned, about 6-8 minutes. Add sliced steak and cook 2-3 minutes more. Season with garlic powder, salt, and pepper.
Add Cheese: Remove from heat. Stir in half of each cheese (3 oz provolone, 3 oz mozzarella, 2 oz cheddar) until just melted.
Assemble: Preheat oven to 375°F (190°C). Spread half the cheese sauce in bottom of 9×13 inch baking dish. Stuff each shell with beef mixture and place in dish.
Top and Bake: Pour remaining cheese sauce over shells. Top with remaining cheeses. Cover with foil and bake 20 minutes. Remove foil and bake 10 more minutes until bubbly.
Rest and Serve: Let stand 5 minutes before serving.