Preheat oven to 350°F (175°C).
Cook manicotti shells according to package instructions. Drain and set aside.
In a large skillet, cook ground beef over medium heat until browned. Drain excess fat.
In a mixing bowl, combine cooked beef, ricotta, mozzarella, Parmesan, salt, pepper, and garlic powder.
Stuff manicotti shells with the meat mixture and place in a baking dish.
Pour marinara sauce over the stuffed shells.
Cover with aluminum foil and bake for 30 minutes. Remove foil and bake for an additional 10 minutes.
Let cool for a few minutes before serving.